How was the fishing? Most times if you have low DO (near the point where you have problems) the stressed fish quit eating. If you had a fair amount of active fish that were eating then my WAG is it was not a DO problem (as in fish kill w/o being caught) but stress from being caught. Were the minnows from the pond or purchased? Purchased baitfish minnows are often hard to keep alive for long.

What were your water temps , and recent rain status ? Did the pond have a green tint?