[Alan Anderson]

For what it's worth, I understood the pictures immediately, and I think they look correct.

How heavy is your robot, and how is the weight distributed? The symptom sounds like the back wheels are slipping a little on the carpet, which suggests that the robot is front-heavy.

The ultimate solution is going to be to recognize that controlling wheel speed precisely isn't good enough to always prevent unwanted rotation of a mecanum drivebase. You're going to have to add some way of measuring the actual robot motion, and in this case I think the appropriate sensor to do that is a gyro.