You know in hotel transylvania when they 'zing' and its like love at first sight or whatever. That's what happens in my mind when bruce gets a new kid.
Seeing Dick sitting in the circus alone and just deciding yep I'm taking this one
Again with Jason, sees a small child, clearly in need, suddenly Bruce NEEDS to take this kid home. Yoink.
He doesn't do it on purpose, or even notice it, just blinks and hes obtained a whole child
He sees kids on patrol all the time, helps a lot of kids, but sometimes his brain just clicks when he sees a kid and then the batfam grows again