This is a great question! Any time there is a great reversal in a story, you can bet there is irony involved; and this story is no exception. A look at the two boys and their dads will reveal plenty of examples of situational irony--a contrast between what readers expect and what they get (what happens).
First we meet Tucker's dad. He is portrayed as a kind, generous man, giving his son whatever he wants for his birthday. Later, though, we discover that Tucker's dad is an alcoholic who bought an outrageous (and virtually useless) piece of farm equipment which ultimately kills a horse--but could have killed a man. What he appears to be at the beginning, what we expect, is not what we discover he is.
Anvils' dad, on the other hand, is an outwardly cruel and unkind man. He comes to school to beat his son, even threatening to kill him. That is all true, and we don't suddenly discover he could be voted father of the year. Yet he has not killed anything or anyone--unlike Tucker's dad. If we expected either father to kill something, it would have been Anvil's dad.
Tucker is a sensitive young man, one who chooses an apple tree over a horse as a gift. He feels connected to the land, and we presume someone who loves the land would also love the people who live on it. Not true, of course. Tucker hates Anvil enough to try to kill him, something we would expect from the bully, not the "good kid." While it's true Tucker was provoked, his behavior can not be justified. It can barely be explained.
Anvil, on the other hand, is nothing but a bully. It's clear he learned that behavior from his bullying father, but he's old enough to make his own choices--and he chose to be a bully. Yet he walks away from the pitchfork incident a shaken and subdued young man. We don't expect the bully to be defeated, at least not so decisively.
When Anvil has Tucker on the ground--humiliated and disgraced and smeared/desecrated with his own apples--there is little to prepare the reader for the deflated bully Anvil will become. We are prepared to dislike everything about him; yet that final image of him walking down the road, kicking his wet trouser leg, creates at least some sympathy in the reader.
Conversely, Tucker's intense connection to land and legacy (did I say he chose an apple tree over a horse?!?) make him a sympathetic protagonist. When Anvil attacks him so persistently and painfully, we almost feel his pain. Yet he deliberately positions his enemy and releases the pitchfork which has already caused one death. We should be glad he defeats the bully, but we are appalled at the intensity of his retribution. Our sympathy has vanished.
Protagonist has become antagonist--and a reader certainly doesn't expect that when he sits down to a story. That's situational irony.