Power outage counts as such (personally I don't think power outage should be the reason to switch over to backup partition).
From what I've seen power outage should not cause partition switch unless the data on active partition is corrupted and boot from it failed.
I mean, just yesterday I've been hard unplugging power from my partitioned RB4011 multiple times because I found a way to make the management plane hang when setting up 80+80MHz mode, and it never failed to other partition because after power on the part0 boot was successful every time.
I think the "boot from it failed" mechanism can also kick in if you have power come back for a very short time and fail again.
(power fail - ... - power back - routerboot starts, begins loading part0 - power fail - ... - power back - routerboot starts, notices "loading from part0 did not complete last time" and switches to part1).