What happened to these vultures?

Some sort of poisoning, perhaps from eating mercury laden fish carcasses or another disease outbreak. Vultures are carrion eaters and have the best sense of smell next to seabirds of all birds. They can smell death miles away and flock to the source.
Still they can be be the victims of secondary poisoning. If they eat carrion that was poisoned they will become sick and die as well.

India used to be thick with vultures, but most are now near extinction. The cause was found to be an anti-inflammatory med commonly given to livestock, but which is poisonous to vultures even in very small doses. That’s likely not the case in the die-off you’re talking about, but it does show how vulnerable vultures can be to chemicals in their food chain.

There is a morbiliform virus outbreak that is killing a high numer of dolphins on the east coast.

They could have migrated to better environments (climate/food). Being ‘animals’ i believe they can sense even acute climate changes that may have occurred in your region. Or simply went after a better food source.

Does this help to answer?

@CWOTUS That sucks. Our poor, sick earth.

I sincerely hope it is not something along the lines of what @CWOTUS posted but you have aroused my curiosity. Please keep us informed if you find out what is actually happening.

Dolphins, bats, bees and now vultures. Very sad.

Actually, reading the link from @CWOTUS, it’s kind of scary.