Scorpion in Injustice : Gods amongs us

Solution 1:

Note: Spoilers for Mortal Kombat vs DC Universe possible

The link between Mortal Kombat and the DC universe is basically the game Mortal Kombat vs DC Universe. However, there is a comic book that is a prequel to the game: Mortal Kombat VS DC Universe: Beginnings.

The story suggests that the Universe of Mortal Kombat began to merge with the DC universe into a world known as "Earth-Mortal Kombat". The merging started with Darkseid and Shao Khan becoming Dark Kahn because of special conditions involving a malfunctioning Boom Tube and Raiden's lightning.

As far as how Scorpion ends up back in the DC universe for Injustice, because reasons.

Also, you can see a list of characters and locations from each Universe can be found here.