vBulletin is a little odd in the way it handles certain things. One is its cronjob process. There's code in every page load, a little bit of j.a.v.a.s.c.r.i.p.t (periods because we don't allow such dangerous terms) -- but let's just call it a gnome. This gnome sits down at the bottom of the page, every page, and checks the time, then checks its schedule of jobs. If it's supposed to take out the trash at 4am, but then no one loads a page here until 4:05am, then that person (the first load after 4) is getting their page "plus." Where the page doesn't complete its load until the server-side processing (the gnome taking out the garbage) is done. A few of these gnome tasks are very server intensive. Which can mean that not only does the page not fully complete its load (and all this means is that everything is there, but your browser activity icon is still animated), but the added stress of the gnome task is also significantly slowing the site down.
Now, there aren't just a *ton* of these, and there are a ton of page loads every day. So it's highly unlikely (slightly less so for our night owls, however) that the same person would be subjected to the same extra page weight with any regularity. However, it's one (of many) possible explanations as to why sometimes some pages load slower than others, but just for very limited periods of time.
The other possible explanations (beyond the gnomes) are illegal immigrants, Ryan Seacrest, frivolous lawsuits (no joke), global warming, dying bee populations, Franklin University, and man-bear-pig.