ESIs usually have already found alternate relationships and tested them somewhat before they make such announcements. They tend to think about leaving months/years before but often don't thoroughly investigate the ship into which they're jumping. Many want out of their ho-hum lives and are good a blaming others for their plights but often don't look very far down the road. I've met a few who've gone from the pan to the fire.
a.k.a. I/O