Their poor performance indicated a fast casual bubble in the market as investors misread the opportunity in the new restaurant sector. One-time market darling Chipotle Mexican Grill ( CMG 0.31% ) may be to blame for that as the burrito roller inspired a number of imitators, but even it had its downfall. Shares got sliced nearly in half after its 2015 E. coli outbreak, and even after moving past the crisis, the company has struggled to return to full strength as competition has increased, doubts among the public remain, and execution of its turnaround plan has often been flawed.