Although swans usually mate for life, individual species and swans may not adhere to this behavior. If something is wrong with one or the other swan, i.e., illness, injury, inability to produce healthy cygnets, etc., then one of the swans may seek another mate.

Are you absolutely sure that the swan being chased is a female and not a male? How can you tell if the swan being chased is the mate, i.e., markings, behavior, etc.?

Is the female (mate) sitting on a nest or in the same area of last year's nest? If so, something may have happened to the eggs, predators, etc., and scared the male or disrupted the normal pairing.

If the female mate is the one that is being chased, we have no idea what may have happened as too many variables that we are not aware and would need to know more information or observe the behavior to possibly pose an explanation. The Regal Swan