This is for sure not an issue with the forums, as you can see others (myself included) are having no issues staying logged in.
Websites remember your logged in accounts through cookies, if you are getting logged out it usually means those have been cleared. If you don't remember clearing your cookies yourself, your browser might also be set to delete all your browsing data when you exit, or it may be deleted by the device you are using. (This is more common on mobile devices, some of which like to 'clean up' app data for a bit of storage space.)
I can't tell you exactly what is causing this for you, but hopefully the above can give you some ideas as to what is going on.