Most of the threads I have seen locked were for one of two reasons:
1 - Other active threads were already covering the topic, IE the new poster had not bothered to check before duplicating a thread; or
2 - Threads were driveby, IE the new poster did not bother to read forum rules about what is required of the original post in a new thread.
Not as frequently:
3 - Thread was started in the wrong forum, for instance The Revolver forum, when these are topics for General Discussion, or Law and Civil Rights; or
4 - Thread violated TFL copyright policy by cutting and pasting entire articles, rather than posting a link or attribution, and quick summary.
Most of these problems would not exist if new guys (and some older members) would just read the rules.