The best I've determined is that on my Mogul it was SPB Pocket Plus. I found a post by another person that said PP was doing it, so I installed the 20273 rom with my normal programs (minus PP) and ran it for a few days, then installed PP and as soon as it rebooted I had the Activesync lockup happen. So I hard reset and installed all my normal apps except for PP, that was a couple of weeks ago and I haven't had Activesync lockup my phone at all, even after many reboots.
And then a few days ago I saw there was a Pocket Plus 4.03 beta available. There is no mention of this issue in particular but I did read they are working on better WM 6.1 support, and so far after many reboots I haven't had any lockups occur. For me at least, PP 4.03 beta seems to no longer cause the problem. In case anyone else wants to try it out you can get 4.03 at
http://www.spbclub.com/forum/viewtopic.php?t=19369