The Sultan has really poor taste in themes. 
Even with FireFox it does this in no pattern I have been able to comprehend. Sign in and start a reply and ------ logged out. Sign in and read a couple and ----- logged out. Log in and make a post right away and I’m safe for a while. Then its all normal for a month or three or four and then back to that again.
Never got any explanation; I blame the Communists. Just for lack of anyone to blame.
Update - I figured out what I did (and reproduced it)
What I did was I followed a link from another board (which was a plain http link). So then I had both secure and insecure board links up on different tabs, because I have a tendency to leave my preferred tabs open all the time and never bother shutting down my computer.
THEN what happened was - I chose a tab to discard, and it was the original one. The http one had been happily sitting there letting me use it for browsing the board and not logging me out as long as I was logged in also with https - it was just when it was the only link up that it started booting me.
So now we know…the rest of the story.
Chrome, to me at least, is the most “paranoid” browser there is. Even our own district web site is marked as “not secure”. LOL
I think that, pretty much, every browser has its quirks, but I like Firefox.