Social movements challenge incumbents and drive institutional change by introducing market alternatives—new products and organizational forms that embody an alternative institutional logic. Research has shown that in response to market alternatives, incumbents resist through heterogeneous behaviors: incumbents maintain their commitment to the dominant logic, effectively marginalizing challengers, while also ostensibly endorsing the alternative logic and often successfully coopting challengers. Although incumbents’ strategic responses to pioneering market alternatives are well documented, we do not know how their heterogeneous behaviors affect new waves of challenger mobilization and how these mobilizations may differently address the hazards of cooptation and marginalization. We investigate the rise of the B Corp (Certified B Corporation) movement against the backdrop of both ongoing shareholder supremacy and rising corporate social responsibility (CSR) among incumbent corporations. Our multi-method, multi-stage investigation reveals that heterogeneous incumbent behaviors encourage new waves of challenger mobilization by seeding divergent mobilizing frames. This variety can lead to a paradoxical form of mobilization in which challengers dynamically balance the tension between their movements’ focus on expansion and purity, rather than prioritizing one over the other. The B Corp movement demonstrates how achieving this balance may help challengers avoid cooptation or marginalization, sustain their challenge against incumbents, and achieve more-transformative change. For incumbents, our findings show that both resistance to and the ostensible embrace of alternative logics may stave off immediate challenges but can also invigorate future challenges that pose substantive threats to the dominant logic.