Objects in group make the entire group move When I try to move or rotate an object in a group, it also moves the rest of the group.

I see something at the bottom of the screen when I try to rotate it that says:

Rot: 0.00 (Smooth) Proportional size: 1.00

I must have accidentally hit a key that toggled this action. What key is it and what mode did it put me in? 

I tried restarting Blender and reopening it but that didn't fix the problem.
Hitting the O key (thank you, @LeoNas) fixed the problem for me. If anyone knows why, please edit this answer.