GFI & storms

[Lampshade]

Since your power was turned back on within a second or 2, it was most likly an automatic reclose that brought it back, automatic recloses are usually in place to "pop" a tree off a line, and they work. If the tree was close by your house, it may have been able to show up as ground current in your house for only a the 1-2 cycles before the fault was cleared off the power line. Usually doesn't happen since there's usually many houses fed by the same circuit, and odd's of the tree being close to enough to yours is slim.

[Lampshade]

may still be a bad GFCI, they do need to be replaced sometimes, that's jsut another option. If it's usually fine and just one isolated incident, could be something external, like the power outage location. If it keeps happening then most defintily replace it, In Manitoba i'm sure you get to test it frequently enough :P. And since you're in Manitoba could actually have been a lighting hit on the distribution system, rare out here, but more common in the prairies. That would defintily raise the ground potential.

[Lampshade]

And... just to overkill this more... Could also have been a surge protector plugged into the GFCI, they shunt excess voltage to ground, which will trip a GFCI. Lightning strike would be a surge, and sent to ground by your surge protector, through your GFCI. Good surge protector's are almost instant, something like 1/100 micorseconds.