To be fair, many right of center groups presume liberals can’t be patriots and that any display of the American flag is a tacit endorsement of far right ideals (which is of course nonsense).

That the US flag is routinely desecrated by reactionary conservatives up to and including police departments in the form of thin blue line, punisher flags, Molon labe lapels etc to explicitly advertise anti-American ideals seems at least as likely to be the cause of growing anti-flag feelings as any liberal agenda.