Well based on the books I've read and speaking to STBXW after we reconciled I know there's a lot of hurt and unhappiness inside them which turns into anger and anger eventually turns into strength to make a change. The only change they think they can make is to run.
However, if there's OP involved then I'm sure that changes the dynamics. In my case there was no OP involved as far as I know...initially anyway.
The fact is that most of their hurt and anger is because they're unhappy with themselves and are trying to make others feel them better. So when despite walking away from everything they find that they're still unhappy that's when it hits them. In this case it doesn't matter if there was OP involved or not. However, they're not willing to admit (their self protection mechanism kicks in) they're unhappy or the fault lies with them- this is the part that can take a long time but eventually it happens. In most cases it's too late by then to make a difference for the LBS.
Me: 35|WAW: 38|D: 6yo | http://tinyurl.com/2dxx7m6 Feb 2006, left, came back in two weeks Aug 2006, left again Apr 2007, filed for divorce Dec 2007, reunited Mar 2010, moved out, filed again