The reason is that they're getting poisoned, and the reason that they're getting poisoned is because there's human-wildlife conflicts. The pastoral communities are using this poison to target predators, and in return, the vultures are falling victim to this.
Grunden er, at de bliver forgiftede, og grunden til, at de bliver forgiftede, er, fordi der er menneskelige konflikter med dyrelivet. De idylliske samfund bruger denne gift til at ramme rovdyr, og i stedet bliver gribbene ofre for dette.