Want to bet? What might work for HF is no guarantee when it comes to 6 meters.
When you speak of grounding the mount, I suspect you're thinking that DC and RF ground are the same. That just isn't the case.
Most likely, the problem is common mode currents flowing on the coax, and if I had to guess, it's the antenna. You can't assume that a 5/8 wave 2 meter antenna will work on 6.
The easy way to tell is, stick a dummy load on the end of the coax instead of the antenna, and see if the distortion is still there.