Just at a guess...
There's a forum software parameter that auto-locks
threads which haven't been posted to in a set amount of time... like say 120 days.
ECF has decided to leave that parameter switched on.
I don't care for it myself, though I accept there are both pros and cons to either position.
I've seen twelve year old (dead)
threads "necro'd" and it can be annoying.
Then again... a few months is rather too short IMO.
It
might be possible to "revive" a
thread by reporting the OP and asking it to be reopened. I stress the "might".
Or start a new thread and hope the original posters see it and are still interested.
I like the first idea better TBH, it would/should alert the participants.