Let me set the stage. 2 gyms about 50-60 meters apart. Both are accessable from the same parking lot and neither require getting out of the car. These gyms are also somewhat close to a main road and during the bit where every gym was level 10, these two flipped regularly.
So I took over those two gyms. I put up a 1331 Santa Raichu and a 1331 Tentacruel. Fast forward 4 hours. I check the gyms. I'm expecting to either still have both or not have either. Instead the Tentacruel gym is still mine and he's gained a friend in the form of a 19xx Vaporeon.
Any idea why that happened? I know Tentacruel is a better defender than Raichu, but I still feel like a 1331 Tentacruel and a 19xx Vaporeon shouldn't have deterred anyone.
A lot of possibilities. Someone hates Christmas, someone only had time to hit one gym. Two players from different teams were playing together and decided to split the gyms - one prestiging and one taking it over, the person ran out of revives, the person ran out of good mons to put in, etc, etc, etc.
I wouldn't overthink it, you'll drive yourself crazy and you'll never know what actually happened.
