Mystery: finding Cayuga duck on back, unable to right itself

We have a 3-month-old Cayuga female. Twice now in the last four days, we've gone outside and found her on her back, waggling her legs, unable to right herself. The rest of the flock -- two more 3-month-old Cayugas, three year-old Khakis, and one adult Buff drake -- stand around her quacking. When we pick her up and set her on her legs, she sits immobile, sort of dazed, and then slowly, shakily gets to her feet, stumbling on her first few steps. Then she is fine!! The first time we found her, when finally she stood again, the Buff drake pecked at her and chased at her briefly. The second time, the drake neither pecked nor chased her.

Could the drake have knocked the Cayuga over attempting to mount her?

We slowly introduced the Cayugas to the Khakis and Buff during the day starting about a month ago. (The Khakis/Buff have been together for a year.) Predictably, the Buff drake chased the Cayugas away from the pool, etc, for a while, but recently the territorial disputes have been much quieter. They still sleep in separate houses, but daytime existence is pretty peaceful.

We have looked in our duck books and online and can't find information on this particular scenario. Any and all wisdom is much appreciated!