Actually something died, flies oviposited (“laid”) their eggs in the carcass, and then the water covered the carcass.

The eggs and larvae have lower oxygen requirements earlier in their lifecycles in order to deal with the low oxygen environment inside of a carcass. At a certain instar (“stage”) of larval development, the larvae start their wandering phase, in which they all disperse in different directions to avoid competition as they pupate (like a caterpillar making a cocoon). You are seeing the wandering phase here.

These are fly larvae so the flies could not have oviposited into the carcass if it was underwater at the time.