I think it stemmed from how all trump voters were lumped into the same category or those who simply didnt follow someone's agenda/beliefs were labeled trump voters.
The phrase trump voter was and is to be used in determining how someone is inferior or dim whitted.
So now to say Biden voters are for masks is kind of a logical step for trump voters. Gotta play by the same rules. Or as I tell my 7 yr old son. If you cant take it dont dish it.