25 is pretty small, especially if there is any sort of delay in any of the queries running. If any happen to need to do some sort of table lock while the mysql server is a bit slow then anyone else on the forums at that point will get queued up.
So, 25 connections could conceivably be taken up in very short order. Normally, even on this site, I'll only see 1-3 active connections. But there are times it can spike up for sure.
That being said, the error you are getting normally seems a bit odd. "Couldn't connect" is pretty generic, definitely not coming from UBB.threads, and the fact that I can get this error on your site when show processlist is showing only 1 active mysql connection is pretty suspicious.
