WMG / My Little Pony: Friendship Is Magic S6 E8 "A Hearth's Warming Tail"

It wasn't eliminating Hearth's Warming Eve in and of itself that brought back the Windigos, it just set off a chain of events that led to them returning.

Hearth's Warming Eve was just the starting catalyst, but if Snowfall left it at that things wouldn't have been as bad. Instead, Snowfall might have been so pleased with her idea to improve productivity by removing Hearth's Warming Eve from the calender that she starts doing it to other holidays, giving ponies fewer and fewer opportunities to come together and celebrate. This leads to a cycle of Gone Horribly Right as ponies eventually become so obsessed with "working hard" and "bettering Equestria" that they lose sight of each other and begin to butt heads about how to best go about things. Cue the Windigos.