My don't know why this is the first time this thought has come to mind, but wouldn't the zombies need water too? We know they need to feed, and their bodies are modified versions of normal humans. We see with the ones that attacked
Michael and
Pegs in the boat that apparently they can starve to death.
Tanya in her studies says they need the same things we need, food, oxygen etc. wouldn't water be one of them?
Could this have been an orchestrated attack? The zeds hear the pumps go on and the windmill is spinning, and
Datu said they had some water starting to come out. Maybe they were laying in wait until they thought the pump was operational, then they jump in and take out Datu and crew in an attempt to take over the water source for themselves? Are the sheds dying of dehydration?