At first, threads were set to close automatically. after about 12 hours, he another feature that will auto start a new thread to replace the thread when it closes. But it was too late for all those previously closed threads. So, we will open replacement threads as you report them here or as we catch them.
It's based on the number of posts, not the number of pages. The number of posts per page can be set in your User CP > Settings & Options > Edit Options > Thread Display Options > Number of posts to show per page.
If you set it to show 10 posts per page, then voila! It will close after 100 pages. The forum default is 15 posts per page, so threads will be automatically closed on the 67th page if you have the default setting.
Again, the reason this is being done is to keep the database tables from locking during intensive queries (many users online viewing popular threads). That is when pages don't update, posts are out of sequence, notifications are not sent for new posts, etc.