Hello,
I did look at the other threads, and I didn't see any that were near as large as the one we moved. Your site should be able to handle them just fine. But a 590 page thread is probably pushing it.
My theory at this point is that the 590 page thread was causing other database queries to backup. It just took so long for the database to query all those replies, that subsequent queries would be held up, causing more apache processes to spawn to serve the the additional traffic still waiting for their pages to load. Of course, this is only a theory at this point, but the statistics are starting to prove it.
My shift is about over, so we'll have to see how this pans out for you. The other technicians on the day shift have the applicable information, and should be able to pickup where I left off if there is any additional problems.
Let us know if you experience any performance issues throughout the day.