Other than a bad tire or tires (even brand new) which can cause a constant pull, the ONLY thing that can cause a car to pull or travel (to the right in this case) is wheel alignment, there is no "something else".
If the alignment numbers are good (as they seem to be) the only components in the rear suspension that affect rear wheel steering would be a toe link or other lateral link that is moving under load because of a bad bushing or mounting hole on the frame end. The alignment is done with all these components at rest. The alignment is good at rest though under load something is moving then you have different alignment values which can steer the car abnormally.