Tailbone rubbing is a common paddling injury and can lead to paddling with pain or missing sessions. The tailbone rubbing on the back of the seat can lead to cuts or abrasions and that's just becomes uncomfortable.
Tailbone rubbing can occur if the paddler has the foot plate too close and they are jammed up in the seat. When you are jammed in the seat and you are rotating in the paddle stroke, your skin is rubbing against the fibreglass. By pulling the foot plate out a notch or two can give the paddler more space to then sit more upright, which brings the tailbone off the back rest.
The paddle shorts you wear needs to have enough material between your skin and the fibreglass.