Completely guessing here, but maybe it's because he's conflicted on whether or not he doing the right thing? Don't read into that too much, just a thought.
I know I'm guilty of doing this, but when plans would come up that I knew my W wouldn't be happy about, I used to have trouble informing her in advance as well. Since things are 'more' rocky between you all, he may be trying to just 'avoid' telling you for as long as he can, so that the repercussions of the event aren't something he'd have to deal with all day long, as opposed to right before he's out in a social environment, where it's easy to forget about them.
I know I mentioned to you before, but, do you still give him forewarning of every last thing you are doing? In your GAL activities, do you always let him know what's going on?
