There could be several explanations:

1. Dietary- Too much protein in the diet causing them to grow too quickly which will also promote the condition called "angel wing" or reverse wing, slipped wing.

Something in the diet is missing, i.e., a mineral or vitamin.

2. Genetic- If this is the case, then there is not much that can be done except see if the cygnets outgrow or compensate for the condition.

3. Lack of exercise in that the cygnets are not receiving ample opportunities to climb or navigate small banks.

4. Injury-The adults may have stepped on the cygnets in the nest or they actually crashed into a bank or other object which could have broken their legs. If this is the case, the cygnets may die as waterfowl do not do well with injuries or even genetic anomalies to the legs.

The best thing that can be done is to get the cygnets to an avian veterinarian and see if the problem can be diagnosed before it is too late to intervene. Good luck. The Regal Swan