This is a mashup of “a tough act to follow” (a difficult performance or presentation after a successful one by someone else) and “follow the crowd” (do what everyone else is doing). The word “follow” seems to be the culprit in this mix up as it appears in both idioms. As John Polk, the submitter said, “crowds are easy to follow, until they turn into a mob.” Truer words were never spoken. A big thanks to “Cliches Gone Wild” John Polk for sending this one in! His logo for his twitter account @clichesgonewild appears below.