Hi, if we don't file, they seem either to file pretty much right away OR they file after a considerable period of time.
My h did much what yours did, only it took him a bit longer to do it, and I think there are quite a few people here who have had the same experience, of filing after a longish period - Sanderika, and Dolphin, I think [forgive me ladies if I have this wrong in your case]. Sometimes the lbs files either to protect themselves financially or because they have had enough. Until they are through the MLC which can take up to 7 years, assuming they don't get stuck! they are not thinking straight. If the affair/separation doesn't solve their problems, then maybe a divorce will do it.
The other thing that seems often to happen is that the divorce then takes a very long time to go through. They delay [often accusing us of delaying] and generally mess around. It is about control, often, as well as a belief, as I mentioned above, that a divorce will 'solve' things.
Sometimes they even think it will wipe the slate clean, and they can start over with us afresh. Truly they are not usually thinking straight.
It is really painful, but gradually I think you will discover that they are still very unhappy and confused people. Take care of yourself. It feels like rejection, I know, and that hurts. I have come to see that my husband is a terrible emotional mess, and although he has hurt both me and my children tremendously I feel sorry for him. Happy people do not do mean and hurtful things to those they loved.