Why? I don't get it

When you and your long term partner call it quits for good why do they move on so quick? Especially when they were the abusive one so they don't even deserve to move on so quick and be happy. And when they move on that quick why do they end up with the total opposite of what they like?

Is it real? Fake? Rebound? One thing I do know is that it is annoying and frustrating because the person who put up with so much for so long, can't move on, because of the trauma he left inside her head