I agree with OP. My H did not have one trigger, but I can pinpoint the one second he snapped. I was yelling at him - now, mind you, this was not common in our M. For the last few months prior to bomb drop, I had been close to being a WAW myself, I was so DONE with being ignored and him being out all night... which in retrospect was the beginning of his MLC, I'm sure... but I yelled "how long do you think we can continue like this before we D?" And that was it, I saw him blink and I saw a mask come on and that was the end of my H.
We were so close, I think the loss of our romance was a huge issue, his children almost being out of the house, as the twins are now 16 and his oldest is 20, and him being over 40 all had something to do with it. I think he began this process back in 2003 as he had an EA back then, but I busted it and we began to reconnect, then slipped and that was that. OW came into his life during his HS reunion, he always had unresolved issues around HS and being shy and not having girlfriends like his friends did, thinking he missed out on stuff, missing/alcoholic father, etc etc.
I didn't do as much as I should have to keep us connected, and in retrospect, had I known what would be facing me now, I would have done more to help him with his issues and stayed his friend/confidant more.