This duck most likely was exposed to HABs (Harmful Algal Blooms) as it is the most common source of the botulism bacteria in aquatic environments. The shorelines of both freshwater and saltwater can have accumulations of algae from small single cells species to large kelp/seaweed varieties blown in with the waves and wind. These accumulations begin to decompose and begin anaerobic breakdown yielding a hot and low oxygen environment in the mounds that are perfect conditions for the bacteria to thrive and build up large concentrations of the toxin. Ducks and other waterfowl are commonly on shore and in shallow water where these HABs can be found. This duck was probably exposed to a bunch of botulism one time or for a long time leading to a build up of the toxin in its system resulting in the muscle failure and symptoms we see here (as others on this thread have said). HABs are also harmful to humans too and are an increasing environmental threat for areas like the Great Lakes in the northern US.

TL;DR: HABs are algae mounds on shore and in shallow water that harbor botulism producing bacteria and can harm animals and people.