I don't think it's useful to categorize liberals in this case, so I'm going to answer the question: Why do [some people] call those who disagree with them fascists

Some people use fascist or Hitler or the holocaust (or the atomic bomb or...) as a generic evil, as per Godwin's law or reducto ad Hitlerum This isn't specific to fascist or WWII tropes, as the term terrorist became quickly misused by right wing interests in the US against journalists and others who sought to serve as a check on the misuse of power by the US federal government, and currently, the term groomer (typically used to talk about people lure children to victimize them) is being applied to LGBT+ people in general by right wing pundits and even some government officials.

However, as Mike Godwin noted, there are actual fascist movements gaining popularity in the United States, Europe and elsewhere. The Republican Party, when it was taken over by Donald Trump during the 2015-2016 Republican Presidential Primary rapidly shifted in lockstep with the transnational white power movement, which has a lot of intersection with MAGA, with the Alt-Right, and with the white Christian Nationalist movement that has been guiding the Republican party since the 1970s, and has been putting Federalist Society jurists on the US Supreme Court (thanks to the influence and mission of Leonard Leo) with the direct intent of retracting civil rights, first from marginalized groups and eventually from the entire US public.

So at this point, a lot more people have an awareness of the threat of fascism, and see all efforts to preserve hierarchy in the US, to withhold human civil rights or to target marginalized demographics as serving to accelerate the fascist takeover of the United States, whether or not the person engaging in the behavior is directly aware that this is the end result. So even if, say, a given person is an OG fiscal-responsibility conservative, by voting for the Republican party (or even failing to vote for the Democratic party) he serves the fascist takeover, the neutering of elections in the US and the rise of authoritarianism. Most MAGAs, for example, are useful idiots, sometimes having socialist ideals, yet voting for Trump-aligned Republicans out of fear of the menaces they insist plague the US (often, imaginary bogeymen, not backed by facts or statistics).