The only other plausible explanation I can think of right now is that this happened at the first commercial break during the Super Bowl, and thousands of local residents flushed their toilets at exactly the same moment. The second wave 3 minutes later could have been caused by their buddies who were over to watch the game with them all going to the toilet simulatenously too... although this is a long shot...
Looks like the backpressure from the storm drain blew off a rather large manhole cover. Quite spectacular I might add. Anyone know where this happend?