Perhaps backup time, or some other event. TBH I don't remember what time they are scheduled (especially in terms of EDT, CDT, WTF - I know they are around 1 p.m. my time), but they do occasionally load the system enough for the forum to stop working. 15 minutes past a full hour looks more or less correct.
Please report if it happens more often.
How long did it take? Shouldn't be longer than a few minutes.