Threads "Locking" after approximately 1,000 posts and new Thread starting.

Does anyone know if there is a rule for this to happen after so many posts? It is annoying when a perfectly good thread suddenly locks and a new one with the same name is created.
I have my posts per page set at 15. So, 67 pages x 15 posts = 1005 posts.
My "Posts for popular threshold" is set at maximum of 999. Has this anything to do with it?

There are limits on Forumotion forums - https://help.forumotion.com/t1709-some-limits-of-forumotion-forum

Here is a non exhaustive list of the approximate limits of the forums. Please note that it is not possible to modify any of these limits.

Messages in a subject: 990 messages (once the limit reached, the subject is locked and splitted)

Hi, as slg said, there are limits. In this case, you have hit a limit.

Don't worry, limits are perfectly normal. There is no way to disable them though.

OK guys. Thanks for the info. Lock and close thread please.