Strange behavior of a rotating drum

Hello,
I'm trying to simulate a rotating drum (primitive y-cylinder wall) with periodic boundaries in y-direction. If I do so, the particles are rolling up the cylinder wall clockwise and anticlockwise (see picture).

I am using the following command:

"fix cylwalls all wall/gran model hertz tangential history rolling_friction cdt primitive type 1 ycylinder ${RADIUS} 4.5 4.5 shear z ${VELOCITY}"

The documentation tells me:

"For zcylinder walls, a dimension of z means the cylinder is moving in the z-direction along it’s axis. A dimension of x or y means the cylinder is spinning around the z-axis, either in the clockwise direction for vshear > 0 or counter-clockwise for vshear < 0. In this case, vshear is the tangential velocity of the wall at whatever radius has been defined. The same applies to xcylinder and ycylinder accordingly."

I am using a positive value for "VELOCITY", so the particles should move in clockwise direction. Does anyone has a explanation for my strange behavior?

Cheers,
Marcel