"Triumph" and "disaster" are two opposite situations of life. One confronts both in his or her life. Both are fleeting in nature as neither of these two lasts forever. 

By "triumph," the poet means moments of success and accomplishment in life, while by "disaster," he implies the time of failure or loss. It’s human nature to celebrate and jubilate at time of success and victory. In a similar way, disaster brings in frustration and despair.

The father is an experienced man and has grasped the fleeting nature of both triumph and disaster. Therefore, he suggests his son that he shouldn't get carried away by either of the two.

They are “impostors.” Both "triumph" and "disaster" seem to last forever when they visit us, but, actually, they pass away soon with time.

If one gets carried away with triumph, it may make him or her conceited and condescending. After success, he or she might find it unbearable to face failures in life.

Similarly, disaster or failures, too, might lead to utter frustration. If one loses his or her spirits and gets despaired, it would make things much worse and extremely difficult to overcome such tough moments.

The poet seems to have seen life closely. He has understood that both "triumph" and "disaster" follow each other incessantly. The best way to deal with these two opposing situations is to remain equanimous and consider these two as "impostors." 

By doing so, one can remain happy in all situations. If one treats both "triumph" and "disaster" as "impostors," he or she would never be deceived by either of the two.   

Why does Rudyard Kipling call disaster and triumph impostors in his poem "If"?

If you can meet with triumph and disaster 
And treat those two imposters just the same;

Truly, triumph and disaster are imposters. To triumph in a wrong doing is not a time to shout the victory. Hitler was triumphant is his murdering of the innocent Jews. He gloated in his triumph for a while. He was quite triumphant and successful in killing millions of Jews. Ultimately, his triumph was an imposter. It was only a matter of time before he was surrounded and defeated by his enemy. Then his triumph was an imposter. His triumph was not real. He lost the battle eventually and ended his life in suicide.

Likewise, disaster, when it comes with all of its fury, may look to be real, but it too can be an imposter. In the face of loss, there can be hidden blessings. For example, one may lose his or her abilities, only to find other abilities become stronger. Joni Eareckson broke her neck in a swimming accident. She lost her ability to walk. She became paralyzed from her neck down. What looked like a disaster was actually turned around for her good (Romans 8:28). Her disaster was turned into a blessing when she learned to paint using her teeth. She would never had learned this had she not faced the disaster of losing her sense of feeling. Her disaster became an imposter because now we have her beautiful masterpieces that would never have been painted had she not lost her sense of feeling or touch. This is how disaster can be an imposter. When a blessing comes out of disaster, disaster becomes an imposter, especially when it is turned around for one's good (Romans 8:28).

Why does Rudyard Kipling call disaster and triumph impostors in his poem "If"?

Enotes provides a fine overview of the poem:  “If” is a didactic poem, a work meant to give instruction. In this case, “If” serves as an instruction in several specific traits of a good leader. Kipling offers this instruction not through listing specific characteristics, but by providing concrete illustrations of the complex actions a man should or should not take which would reflect these characteristics.”  It is interesting that he personifies both “Disaster” and “Triumph,” and capitalizes the words to call attention to this.  Kipling also personifies “Will” toward the end of the poem.  Significantly, something might look like disaster but not be so, or might look like triumph but might be something else (such as defeat). For these reasons, they—the experiences of disaster and triumph—might be “imposters,” not really what they appear to be.  “Will,” however, is unmistakably that; it cannot seem to be anything other than what it really is.

Why does Rudyard Kipling call disaster and triumph impostors in his poem "If"?

My thinking is that the speaker wants us to view life as a continuum, marked by peaks and valleys that may or may not be seminal events in our lives.  If we can resist becoming too self-assured by our successess nor too defeated by disasters, we can live more contentedly.

I am reminded by of the quote by Golda Meir, "Don't be so humble; you're not that great."

Why does the poet say that triumph and disaster are two imposters in the poem "If" by Rudyard Kipling?

Over the Wimbledon Tennis court tunnel which takes the players back to the locker room  is this phrase:

If you can meet with Triumph and disaster

And treat those two impostors just the same.

What a wonderful poem! It is one of the most beloved poems in literature, and its message is timeless. In the poem, “If,”  Rudyard Kipling gives advice to his son on how to become a man; yet his advice rings true for everyone.

This poem is labeled a didactic poem because its purpose is to teach.  In each stanza, Kipling provides guidance in some aspect of life.

The first stanza covers building self-confidence, never giving up, not judging other people too harshly, being patient, and loving not hating.

In the second stanza, the poet’s instructions include always dreaming, using his  intelligence, and ignoring fools. 

The two lines above the Wimbledon tunnel are found in stanza 2.What do the these lines mean? Triumph  signifies  winning, victory, success, and achievement.Those words are easy to live with.   However, as the adage states, Winning is not everything.  If a person does win a tennis match, the spelling bee, the beauty pageant--never boast  but show sportsmanship toward the fellow competitors.  Doing the best a person can do is winning no matter what the outcome.

On the other hand, disaster brings a different set of circumstances: tragedy, adversity, loving, misfortune, and defeat.  Not situations that anyone finds comforting.  With this idea comes losing with grace, remembering that he did the best he could do--  then there is no loser.

Kipling does personify these words.  Personification ascribes human qualities to something.  Here these two aspects of life are given the ability to be imposters.  They are pretenders because both situations  are fleeting [They do not last!] Winning is great, but it is only temporary.  Thankfully, disaster is momentary as well. Everyone wants to win,  and nobody wants to lose. It is the grace that one shows in either situation that makes these imposters ludicrous.

Remember that the  tennis players know both triumph when they  win, and disaster when they  lose.  There is only one winner and many losers. Each year, the athletes keep coming back to play the game that they love. These people know that it is not whether you lose or win, it is how you play the game.  Kipling ends his poem with this  assurance:

Yours is the Earth and everything that's in it,
And---which is more---you'll be a Man, my son!