I have no idea what causes it, but it's really annoying. I do have saved logins on two different computers (maybe that's relevant) but I'm never using both at the same time. I also open multiple threads in multiple tabs (I assume everyone does this). Sometimes I can't even post because every time I try to reply, the forum says that I must be logged in.
Ah, I figured something out -- I've been going to http://cardhunter.com/forum This currently redirects to the https version of the forums.