Let's just say something like oh...

Caravan was about to be attacked by I dunno angry bandits. During that time Boom was still running away from enemies, which he didn't know long since lost the track of him. But he sorta assumed they were still chasing after him. During his fleeing he tripped and accidentally sent one of the Bombchus he was holding onto flying into the bandits, and it killed/harmed and terrified any living bandits off.

So he played off of it as something he totally intended, in order to get hired and more importantly protection by the Caravan. 

How's that?
Any other background details you want?

For like 'as a person'. Survive by any means necessary. Kokiri law son.