There are several reasons that this maybe happening.

These may be juvenile male swans and they are trying to mate with the female and are too inexperienced to know they must hold her head above water to prevent drowning. Although this could be a reason behind the behavior, it seems unlikely at this time of the year as it stands not mating season until springtime.

The more likely explanation is that the two swans are adult swans and the one being beaten is a juvenile swan who has encroached on the pair's territory or got caught between a fight between two males over territory.

In any case, if the swan cannot escape and this behavior continues, it could drown or suffer severe injury or stress. You might contact someone at Abbotsbury Swannery in Dorchester to see if they have someone who might intervene and rescue the swan if necessary. You might also contact the RSPCA or swan rescue group in your area. They may be able to come out and evaluate the swan for any stress or injuries. The Regal Swan