I am afraid but this issue is caused very likely by a static discharges.

In the Spirit flight log you will maybe see error messages about your receiver signal.
Also in the Diagnostic tab you will probably see how the channels are jittering.

This issue is not related to BEC.

If you are going to ground the tail system, ideal solution is to ground it to your avionics (receiver) battery negative - if you have one. This will allow current to completely bypass the avionics on your system as it goes to ground, while also protecting those avionics. Covering open, empty servo ports on electronics with a small piece of electrical tape also works very well to prevent a static discharge arcing to the servo pins.

If you do not have a "avionics" battery, only a motor battery (because you have an onboard BEC), then ground to the motor battery negative terminal.

Grounding to a motor is not ideal at all, as a motor does not maintain a constant ground (it's isolated as the motor phases switch). A large, metal motor plate can work, because it acts like a ground plane, but going straight to the negative terminal ("ground") directly is far, far better.

Not sure what material your frame or tail boom material is, but remember that if it's carbon fiber, you must remove the clear coat / epoxy completely at your grounding points - it is a very good electrical insulator! An intermittent ground contact in some cases is worse than none at all, as you can end up with horrible RF noise.