And...that's the part that makes less logical sense.
Them monsters should be sleeping in their caves or something in the winter and start popping out during spring hungry and roaring for some juicy peasants to eat. Then again, maybe these are arctic monsters like polar bear mutants that move in the winter. Or maybe monsters are actually migratory predators. Hmm...
Autumn is when food starts getting scarce and they need to make provisions for winter. It's the *most* logical season for creature that hibertate.
Then again, who says they hibernate? Spawned monsters don't disappear. Therefore, winter would be the most logical season because food is scarce in the wild and they must roam to civilized lands for food.
It could also be argued that summer is the most logical time for them to spawn, as food is abundant and they can breed all they like.
Really, you could argue for any season, because we don't have the slightest idea what the heck a "monster" is. I'd personally go for winter as being the most logical, though, for the reasons I stated. They don't appear to hibernate, so they'd be more likely to go out in the open when food is scarce, like wolves and coyotes do.