To answer your question, Byrdie -
Each and every moderator has their own methods as to how they do their job. It is to it's own. We are not required to make note of why we close this said thread.
As for me, I always make note of why I close this thread in order to make it understanding as to why the said thread is closed for. That's just how I am.
For instance, If the OP makes a post in the said thread that requests his/her thread to be closed; That alone is a reason why we don't have to make note of the said thread that is being closed.
If a thread turns out to be ugly, That is another thing as to why it would state the obvious reason to see it being closed.
Again, It is to it's own. Also, Some things are better left being unsaid too, if you know what I mean by that.
Does that answer your question, Byrdie?