I don't think it is based on their "own discreation" rather its based on forum rules.
Me neither, and if mods could or did lock threads based on factors other than forum rules, a shit storm of chaos would probably ensue. My guess is that most of the threads they lock are garbage ones that should never have been started in the first place or were ones that got reported by someone for breaking a rule.
As far as mods giving a reason why, I don't think that's ever going to happen even if I thought it was a good idea, which I don't. They've got way too much to handle as it is without having to justify their actions when said actions usually aren't controversial in the least.
When a thread is a spam megathread it often gets deleted as deliberately inviting spam could be considered against the rules.
But it also does happen that mods will lock an old thread at their own discression without it being spam or breaking any rules. Not everybody should be up at arms with this and also it wouldn't be too much work to simply add a simple response to a thread before locking it to justify the decision.
The best option would be for Bitcointalk to have a drop-down menu so this can be done automatically when mods lock a thread but I don't think it makes sense to request features for the forum based on wnsy we've seen so far.