There are a number of reasons why a topic is locked.
As Otherclive has said sometimes it is because a very old topic is resurrected, usually by someone testing the system in advance of spamming the whole Forum.
Other reasons are when topics become too heated and the topic results in untoward personal attacks on other members, or when base language is used which breaches the rules.
The Forum is read by all ages and in that respect, moderate language is expected and strictly enforced.
Locking topics is not taken lightly and every effort is made to avoid doing it, but sometimes this approach is ignored and that is when it gets locked.