Ok...it wasn't life altering or anything...I was just curious why it was happening so often on this site and nowhere else...so basically no one really knows for sure. No problem.
I know 'for sure'....as I said 'cookies'.... 
When a site reliant on a browser's history of interaction with the site is updated/revised/rebooted the cookies would need to be rewritten/updated too.
Tracking is used to let the site show you specifically whether you have already read a thread...or not, etc. If the cookies are prevented from updating or are deleted through expiry [and not allowed to rewrite] the site will fail to recognize you...and one obvious result will be your sudden logout...