Sounds like a digestive impaction that has resulted in slow digestive transit and therefore food in the crop has built up and gone sour. They drink a lot of water to try and clear it themselves. Effectively the bird gets poisoned, rather than dying of starvation. The usual cause is putting hens onto grass for the first time when they have empty crops. We lost two like that, because in the temporary shed they got bullied off the feeder and were put out hungry -of course they gorged themselves on grass and impacted their gizzards.
So if you sold hens last month Marches Farmer and they were reared on soil, then the new owners put them straight onto grass, that may be the problem. The timing is about right perhaps -ours died two weeks later.