1) Some of them were protesting other regulations and mandates.
2) The stay-at-home orders caused many people who would otherwise be working (and during the springtime, planting) to not be doing what they would normally do. That's going to frustrate people. The initial protests were about that, and the mask orders came in, and so all of that frustration about not being able to work and plant was able to crystalize under the symbol of people wearing masks.
3) The American people were told initially that masks were no good out of fear that hospital workers wouldn't have enough masks. While there is such a thing as a "noble lie" it was still a lie, and people remember that and have less trust than they already had in "authoritative" sources since the authority lied.
4) The movement is bottom up, not top down. In Michigan, where much of this started, there was a newly elected Democrat governor who would not be up for re-election in 2020 because her four-year term started at the beginning of 2019. So this is a governor with plenty of time left in her term with no one running against her yet. And the protests began because her policies in response to the pandemic seemed to be more severe than many people believed was warranted. They angered enough people throughout the state to come to one place, the state capitol, and protest when the objective of those orders was to keep people at home and distanced. She issued orders that made people with a lot more free time than they usually had, upset.