If anyone else had the "reboot into activation" issue, I think I found the culprit.
Background - I'm on the 4.0.4 radios, rooted, rommed, etc. etc.
For about a month or so my phone would randomly reboot into activation. Ordinarily I'd write this off as a random reboot issue, but the "activation" part bothered me. So I decided to pay closer attention, and here's what I found. Keep in mind, absolutely no scientific methods were used here, simply observation and relentless OCD.
When switching networks, e.g. WiFi to 4G, etc., there appears to be a "force signal" type thing going on in the background. When you're switching networks using something that's a "heavy" data user, like 90 million Chrome tabs, Spotify, etc., it appears that because the app is demanding or using a heavy amount, the device will reboot in order to force the signal to take. Much like is recommended here and elsewhere ("rebooting your phone should turn the bars from grey to blue, etc."), the phone seems to have a mind of its own.
Does that necessarily solve this issue? Not at all. However, it gets to the point where I could literally force it, and to me, being able to reproduce it is much more reassuring than having no idea what is going on.