Basically, vultures were very important in certain areas to remove cadavers from the area - such as countless cows raised by lacto-vegetarians. ...
An anti-inflammatory medicine was introduced about 3 decades ago to help treat cows (probably helps with milk production).
The substance stuck around in cows who died and were dumped "outside" or "elsewhere", as trash is often dumped.
Vultures ate dead "tainted" cows and also died.
As vultures died, other carrion eating animals came by and feasted, but they were not as nice as the vultures for the ecosystem and for the humans in the area.
Note that vultures often die because of eating poisoned animals, poisoned by humans. Lead, for example, is a common deadly poison left behind by bullets from hunters. Many animals deemed "pests" are poisoned and end up being eaten by a vulture who dies later. That's aside from the various poachers just outright killing vultures.
