Well, once they start to happen fear can spread if they are extreme and the low statistical chance can be hard for people to appreciate.
I am more aware than most since a close relative actually had one of these rare reactions, we don't know it was linked to a vaccine but it was something that has been linked to vaccines in some cases. That said, neither she or I are anti vaccine at all and we know these extreme complications are more often a consequence of the disease than the vaccine.