Along with what Titan said , Its based on a couple things - Most of which is criteria only known by the m++ and above.
Some of which include:
1. If the topic is not on topic, or has off topic posts that have potential to start a problem (Or 'flame fight')
2. If there already is flaming, or fighting between members.
3. If its in the trading grounds, a post above 60 replies I think gets closed... Or is this an old rule? I would refresh yourself on the rules - I have to myself

A lot of it is up to the wisdom of the person whos closing it, if they feel it is a problem, or will cause one - They will close it

Thought dont forget this is the Site suggestions thread, so your support to post suggestions - Not ask for help.. That belongs in General Help or Chat maybe.
What you could do to go along with this (a great idea if I say so myself :P) is think of some criteria YOU would close a thread for, and try not to do those things


Have a good day