We ask why most online signature campaigns attract only a handful of supporters while a few are backed by millions. Using a field-experimental design on the popular petitioning website Change.org, we investigate the role of emergent phenomena during the mobilization process in determining levels of collective action. We demonstrate the significance of these emergent processes while controlling the structural profiles, organizing strategies, and initial signatory volumes of online petition campaigns. Differences in ultimate signatory support among similar petitions are as extreme as theories of critical mass and tipping points would suggest, yet the dynamics leading up to these arbitrary disparities are unexpected. Growth is highly erratic, with initially unpopular campaigns experiencing wholly unexpected revivals, limiting the predictability of future from past momentum to just the short run. The mechanism we identify driving these unpredictable dynamics we call “accidental activation,” the unanticipated recruitment of secondary mobilizers.

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