Why did I get an incubator from a Pokéstop?
I was driving home and stopped by a Pokéstop to stock up on balls and potions, but I was surprised to get an Incubator among my loot. Are these now in the Pokéstop reward pool?
This is part of the Christmas event, described here:
Starting the morning of December 25, 2016, and through the afternoon of January 3, 2017 (PST), PokéStops will award one single-use Incubator each day after your first Photo Disc spin.
Thus, you'll be able to get one Incubator a day for a full 10 days during this event, which will allow you to hatch a single egg each!