I have the same problem, even though I have the button checked to make sure I do stay logged in. The site has apparently been having a few issues in general though lately so it might just be a bug. Hopefully it's only temporary and it'll be fixed soon, though it's not a super huge deal if it isn't.
If you are using Firefox there is a new security update that does not trust any address that does not include http:stuffstuffstuff and automatically logs you out. The current forums are within the security criteria for being shady.