Looks like you have a misalignment further down in the robot. Delete the misaligned parts and rebuild. Using the “auto” snap mode (shown below, highlighted in gray) helps to reduce misalignments.
Further to @Sam’s point. The Mimic editor tests for collisions when running. We saw in your example project that there were some shafts that were sticking through the other model elements which would result in a ‘collision’ and thus cause your robot move on its own.