Sorry to hear about your shrimp. If it wasn't a contaminated bucket, or chlorinated water, the only things that I can think of is that the water temp was way off, or that there was an abundance of nitrogen in the water that commonly happens in winter.
I meant CO2 dissolved in the tap water. Sometimes there is a lot and if you have plants you will see them start producing a lot of O2 after a water change. As said before could the temps have fluctuated?
I not an expert by far but like what others have mentioned, I think it a contaminate. Could be from the bucket or even maybe something that you got or spilled on your hands.