Like any electrically driven system, a flicker of the power could have caused a reset and unintentional opening. Shouldn't do that, but its possible.

Several years ago our garage would go up or go down randomly. Come to find out our neighbor a few houses down on the other side of the street had the same problem. They started walking up and down pointing the opener at each garage and ours was the one that opened. They had a new opener put in and it was the same frequency! If they were close enough to our house when they pressed theirs, our door went up or down and if we were at the correct angle theirs did also. They had their installer change the code. If it keeps doing it, you might change your frequency.

I used to do production subdivision garage doors. Every single lot got the same opener. You could drive through there mashing a new uncoded out-of-the-box remote and watch the doors go up all over the place. Installers on piecework are one lazy bunch...

It may have just been a glitch with the thunderstorm, we had reset/used the learn button from the previous owners, so i doubt a new garage opener somewhere did it, no one around has gotten a new door recently. Hopefully that was all it was and nothing more.