Both? People are very easy to FUD up about any local development, and they rarely bear any costs of denying those development.

Being eager to uncritically lap up any FUD thrown your way is definitely a personal failing. Let alone spreading that FUD. But the fact that NIMBYs are allowed to wield an outsized power and do things like stall nuclear power for decades? That companies can engage in underhanded "find an activist group that wants to NIMBY in an area where the competitor is trying to build and then just fund it"? Failure of an incentive structure.