Here the word "choo" is short for what you, with the auxiliary verb omitted. This line by Stan is basically:

"What (did) you fall over for?"

Dropping the auxiliary "did", what did you becomes what you, then whachyou, then choo. Stan is asking, "What did you do that for?" "Why did you fall over?" That is why Harry responds by saying, "I didn't do it on purpose."

I found a page with your quoted text. If you go down a bit further in the text, you will see another instance of this grammatical phenomenon.

"Choo lookin' at?" said Stan.

And this line means "What are you looking at?" with the auxiliary "are" dropped.

Here the word "choo" is short for what you, with the auxiliary verb omitted. This line by Stan is basically:

"What (did) you fall over for?"

Dropping the auxiliary "did", what did you becomes what you, then whachyou, then choo. Stan is asking, "What did you do that for?" "Why did you fall over?" That is why Harry responds by saying, "I didn't do it on purpose."

I found a page with your quoted text. If you go down a bit further in the text, you will see another instance of this grammatical phenomenon.

"Choo lookin' at?" said Stan.

And this line means "What are you looking at?" with the auxiliary "are" dropped.

Also worth noting that two more common contractions for "what are/do you" are watcha or whatcha.