The incompetent humans do not know that they have too many dogs, so they overfeed and then underfeed them.

Charles and Hal have no idea what they are doing.  To them, it makes sense to have as many dogs as possible tied to the sled so that it can go faster.  Unfortunately, they do not realize that the more dogs you have, the more food you have to have.

In the nature of Arctic travel there was a reason why fourteen dogs should not drag one sled, and that was that one sled could not carry the food for fourteen dogs. But Charles and Hal did not know this. (Ch. 5)

Mercedes feels for the dogs, and wants to give them extra food.  Unfortunately, the combination of feeding the dogs too much and not having enough food to begin with leads the hapless humans to go from overfeeding the dogs to underfeeding them.

Then came the underfeeding. Hal awoke one day to the fact that his dog-food was half gone and the distance only quarter covered; further, that for love or money no additional dog-food was to be obtained. So he cut down even the orthodox ration and tried to increase the day's travel. (Ch. 5)

Thus, while the underfeeding was inevitable, it came much more quickly because they fed the dogs too much at first.  Underfeeding is very dangerous when dogs are running all day and it is freezing cold.

As a sled dog, Buck finds himself facing a long line of cruelty by humans.  It is ironic that Mercedes tries to help the dogs and ends up making things very uncomfortable for them.  Since the humans do not know how much food to pack (or that it is impossible to pack enough food for fourteen dogs), they run out.  Some people always think they know better than the experts.  Charles, Hal and Mercedes just did not take anyone’s advice.