Why did people start falling unconscious in "Futari wa Pretty Cure"?
Talking about the very first season of Pretty Cure, Nagisa and Honoka had to face Zakenna, black monsters that come out in form of shadows possessing objects and creatures. When those monsters appear, people fall unconscious while the sky become dark.

AFAIK, since Zakenna appears only in season one and season two (Max Heart), the behavior you're asking for seems to be related to Zakenna particularly.