Some horses just swish their tails to show their protest. Others might bite the bit, pin their ears, stomp their feet, etc. It's just their way of expressing themselves.
My quarter horse swishes her tail. Now, in the AQHA circuit, we are allowed to put weighted tails in and that stops her tail from swishing. I got my tail from Kathy Williams in OH.
If dressage doesn't allow weighted tails, maybe put one in while you practice before your event. Your horse will try to swish with the weight and get tired. It's like lifting weights and your arms get tired. Your horse wont want to swish her tail in the event.
I hope this helps.