Loops have so much topspin that returning with backspin will continue the ball's spin direction and lift it high up into the air. You therefore need to counter loops with an attacking stroke in order to reverse the topspin and bring it back down onto the table.
The ball trail, the ATTACK button and the screen edges will glow orange to remind you.
👍 TOP TIP
Players with a Mega Chop can backspin Loops. This is because this chop generates enough backspin to get the ball back onto the table.