I was guessing a sudden cold snap would do this -
it’s what caused a massive fish kill in Fla a year ago.
The birds that died would have been the weakest members of the flock. i.e. If the flock numbered 50,000 and only 1,000 were lost to ice, hail, or lightening - that would be a very small percentage.
Somewhere, the remaining birds are flying around - eating and roosting prn. It would be important to find the flock to determine the exact cause of the dead drop-outs.