A leak!!

Try aiming a hose at the rear, from the top of the tailgate, and have
someone inside watching for water entry.
Usual suspect points are the tailgate seal, the rear lights or a badly
made body seam.
Maybe the glass seal or round where the wiper comes through the glass.
It may take a while for the water to get through where ever, so maybe
rest the hose on the roof, aimed at the rear. DON'T squirt hard! Just
gentle running water should be sufficient.