http://www.newsweek.com/pokemon-go-gen-4-update-eggs-1173123

Whenever Pokémon Go receives a big update or introduces a new region of Pokémon, the types of monsters that hatch from eggs changes.

The Gen 4 Pokémon Go changes what trainers will find in 2, 5 and 10km eggs. Niantic is making it a bit harder to find Sinnoh Region Pokémon this time around. Pokémon from older generations will still hatch from these eggs, so what you will find will be up to random chance.