Packers v Vikings - incomplete pass reviewed

[Dakota]

This is NOT a fanboy question... I have no rooting interest in either team.

Did anyone see the incomplete pass by the Vikings that was reviewed? The call on the play stood (incomplete pass).

The slow motion instant replay showed on the broadcast showed (to my non-football official eye) that the Viking player caught the pass, had it cradled in his arms (apparently in control), he was hit (hard) by the defender, he went down (not a tackle - he was knocked to the ground by the hit), and the ball came loose upon contact with the ground.

Why was this an incomplete pass?