Threads get locked generally cause they have gone to hell in a handbasket, it has nothing to do with how many replies or threads on a given topic. For example, there is one i can think of that has 80 posts and hasnt been locked. There are tons of threads on the same subject that arent locked.
When a topic is inflammatory and shouldnt be prolonged, or many posts are made that are nothing more than flaming or bickering, (or useless nonsense done in the name of boredom) a lot of the offending posts may be removed and the thread locked to put a stop to it. Before you suggest asking those involved to stop it rather than lock the thread: been there, done that, sold out the video, wore out the tshirt, didnt work.
Other situations i can think of when a topic may get locked:
- if the poster has crossposted, and ppl are trying to keep up with what has been answered on all of them. Thats just wasted time and annoying. PMs asking the poster to stick to one thread and stop driving ppl nuts are usually not productive.
-if the thread has gotten so offtopic its ludicrous and has gone into a bunch of posts on something like why arent bananas blue.
Some stickies are locked simply because they are for informational purposes only.
I realize that when posts are deleted, anyone who hasnt seen them may not understand why the thread was locked (you can thank us later for sparing you) and of course you arent seeing any pms or moderator discussions that are flying about. I guess all i can say is that in those cases, trust that there is a good reason.