I have 7 ducklings, still with their mother, and I watched one start to go twitchy, like in your video, and then it recovered several minutes later and full locomotive abilities to run and keep up with the rest. I too wondered if it was a seizure. The mother struck it on the neck while it was twitchy (I assume to separate it from the rest). This doesn't seem to have anything to do with bumble foot. It appears to be neurological to me because it went from normal active to twitching and spazzing and then back to normal activity. But I join you in the confounding question as to what is causing this!! I'm not sure B3 is quite the answer.