Something like "safety in numbers" effect, as, I would guess, (after some threshold that provokes government's action (maybe not before)) the greater the number of protesters, the lesser average danger an individual faces (as the perimeter (i.e. the most dangerous place) of the geometric shape of the crowd probably grows slower than the area (the number of them)). Furthermore, the bigger the crowd, the harder it is to track their identities, and they might also expect that the government might be unwilling to punish all of them (rather than just the leaders and a small number of others). In addition to that, the greater proportion of population joins these protests, the greater peer pressure for others to join as well. I would guess that once the most risking taking individuals start everything, it becomes easier for others, who support them, but wouldn't start the protest themselves.