Has someone been cancelled as a purely grassroots effort before? It seems like the seeds of a cancelling may begin this way, but quasi-journalists, pundits and other online personalities are the engine that does the crucial amplification to reach broader public discourse.
I think their is an interesting sort of 'laundering' of attribution where social media power brokers can point to the seeds and say "Look at this grassroots movement that is changing the way society views X", and then the issue is broadcast to a much wider audience. However, that small grassroots movement may not have been seriously affected X except for the effects of the power brokers' pointing fingers.
It seems like these social media power brokers may not even intend to have this effect in some cases, but they are a crucial step in the cancelling lifecycle.