Answer
This is the result of a recurring bug in Pokemon GO, that often hits when Niantic rebalances a Pokemon’s stats. As mentioned in the link:
It will fix itself as you hatch all of your eggs obtained before [a] stat rebalance. They just forgot to update the current HP of pokemon still in eggs when the rebalance hit. You’ll just need to heal or toss the hurt pokemon that you hatch.
Attribution
Source : Link , Question Author : Dragonrage , Answer Author : Lamprey