The attacker ran into Lovren, not the other way round. He then milked the collision for all its worth. All Lovren did was put his hands on him and he went down. It's not a penalty. Or it shouldn't be. Everyone saying it is just contributes to the diving mentality we have in the game.
The attacked didn't run into Lovren at all. He collected a pass and shielded the ball from the defender chasing him down as he tried to fashion a chance on goal. Lovren just ran into the back of him, with both hands on his back. It's a foul every day of the week, and as it was in the box, resulted in a penalty.
Players shield the ball of the time, and the correct way to defend that is to get tight without putting your hands on their back. It's not diving when the person behind you charges into you.
Diving is simulating when there's no contact, or contact so minor that it wouldn't even knock you off balance. Calvert-Lewin took the ball forward, then slowed down to shield it and change direction, presumably to turn and see if there was someone he could lay it off to. Lovren didn't slow at all, and instead ran into him at full speed with both hands on him.