The main street bypasses are very common to open when its busy, has nothing to do with power outages.This happened to us 2 years ago - - - - The WDW power grid was struck by lightning and shut down a ton of MK rides.
In this case - - - the 3 other parks were actually closed due to this power surge.
So MK has a surge of people after 5pm......It was insanity.
At close they opened up the cast backstage exits........it was cool to see the backstage of Mainstreet USA.
Crappy though to ruin the mood when exiting.
Hopefully next time will be different.