That toggles the BBcode on and off ... if you're of a programming persuasion, you can type the relevant code in square brackets yourself. Or, click the [ ] and rather than typing [ B ] for bold and ending with [ /B ] you can just click the B button.
There'll be a cookie to remember your choice - it might be a cookie per sub-forum which might explain some of the randomness. Maybe you have a privacy blocker in place which rejects those cookies. Dunno.