Without specifying a specific incident, it's hard to give an answer.
That said, there are many reasons for closing threads. A downward spiral of incivility or a prolonged drift off topic can do it. So can technical issues, such as threads in the Gun Show that don't adhere to the rules. Sometimes, there are technical reasons for doing so.
Rich Lucibella, the founder of this forum, gave us a vision of what it should be, and that's embodied in the rules.
Sometimes it’s nice not to destroy the world for a change.