In the relocate project, it's pointed out that Bon is more broken compared to the other animatronics in the facility, as if someone broke him "on purpose."
I honestly have no clear idea what could've done it, I do have some theories, but they're just speculation and trying to make sense of something I don't really understand.
Not really an important theory, but I believe Brian Stells was injured in some way or form that impaired his movement/speed, which allowed Bon to catch up to him... I think.
I mean, I believe running away from Bon isn't really much of a problem, David did it, and I don't believe an animatronic with not so good pathfinding can catch someone sprinting away.
And when Bon caught Brian, in the flashlight scene, Bon already had some blood on him and his claws, which means Bon did catch Brian someway, but Brian managed to escape (?)
Just to be caught again.
I don't know, this might be obvious to people, maybe not, I'm just yapping.