Four year old and his unusual lies.

[WeAll]

My son will be five next week, he has this awful habit and keeps getting himself into trouble with it.
When he bumps into someone accidentally, he says "I did that because the other boy was not sharing" or "I did that because the girl wouldn't give me the bike when it was my turn". He invents a motive and I have seen him do it when it was definitely definitely an accident, but then everyone gets cross with him because he talks as though he planned it! I am 100% sure he didn't!
Why is he doing this?

I am worried about at school, when nobody does see what happens.
He makes himself sound like a horrid boy!

Making up stories at other times might be a good idea.

Thanks BeerTricksPotter. :)