It's caused mostly by someone actually using the search feature, and the system having to hunt through millions of posts. It would be nice to have a dedicated search engine that wouldn't hog all the resources.
Oh, and people not pressing "post" 4 times in a row because it didn't instantly update. Kind of like people at work hitting "print" for their document, not seeing it immediately come out of the printer, hitting "print" again and again, then ending up with 4 copies. I guess those people also hit the elevator button a dozen times too, because it makes the elevator come faster. 
In my case, you get duplicate posts because the system won't respond after you press the "post" button. As an experiment, I posted this thread and walked away for an hour, came back, it was still working away (or so it said). So, I quit out of the post, then refreshed the thread and my post showed up.
Searches alone shouldn't bog down the boards this much. Any time anyone refreshes any one of the boards, it queries the database again. Imagine a hundred people doing this over and over all at once.
Yes but SMF is downloadable and I don't think Skuzzy can do the backend stuff that he does outside of the forum admin CP itself unless he has access to the core files.
SMF is, yes, but from what I'm seeing on the site linked below, it's a hosting service that runs SMF. Irregardless of whether or not that is true is kind of moot anyway, the point is that the server doesn't seem to be able to handle the load of this community anymore.