They're suspicious of his motives behind pushing vaccines so aggressively. One of the points of contention is that Bill Gates has been known to publicly express the idea that the world's problems are due to overpopulation and that depopulation is the best solution to it. So why would a man who is in favor of depopulation actually push for vaccines? Vaccines would SAVE lives which wouldn't decrease the population.
On the other hand, those who aren't suspicious of his motives believe it would be near impossible to pull off an evil scheme like killing millions via vaccines without someone somewhere down the chain of command blowing the whole thing open.