Threads get locked for a number of reasons:
For being about restricted topics (real-world politics, religion, other "hot-button" discussions, etc,) for re-posting of a previously locked topic, for discussing moderator actions or otherwise bringing up discussion not appropriate for the open forum, and also for being too full of flaming and other rules violations to be worth a moderator's time to clean up. (A full explanation of all this can be found in the forum rules pinned in every sub-forum - they probably do a better job of explaining this than I can.)
We don't expect the various Issues forums to be full of happy excited and calm people (heck - that's usually too much to ask for the
General Discussion forums!

) We do expect, however, for everyone to follow the same set of rules that apply to all members regardless of if they're posting in a General Discussion, or an Issues thread. These same rules apply to the 360 and PC Skyrim Issues forums, as well as all of those for Brink, Rage, Fallout: New Vegas, and so on.
This essentially comes down to not making duplicate topics of existing active threads, avoiding posts which insult or are otherwise rude to other members; and to report any posts which violate these rules instead of responding in kind.
A flame that gets reported means simply one or two posts that I have to remove and a couple of people that I would need to deal with. Responding to a flame with a flame that's just as bad means double the work I'd have to do to clean up a thread - more only compounds the issue. And that's why threads get locked.