Some are because they want to have more romance/ admiration/ respect.
Some because there are other underlying problems in the M.
The reasons can be so varied, but from what I have read in the past - and I can't remember which book I read this in, some have exit A's - the intention was always to leave the M and an A was a way of getting their spouse to do the work and kick them out - that way the WAS doesn't look 'quite' as bad perhaps.
Your H's A doesn't sound like an exit A though as he has no clear plan as to what he is hoing to do - as I understand it, usually those having exit A's usually have a pretty solid plan.
Saffie me 46 H 46 M in 1986 D20,D18,S16,D13 H's A 01/05 to 07/06 H recommitted to M 07/06 renewed vows 09/06 Going from strength to strength