In cases of this type, it's an error akin to relativism. "Suzy is wrong, therefore I should kill her" does a lot of damage, so people start thinking "Nobody is ever wrong". "Members of outgroup X have negative traits, therefore they're subhuman" does a lot of damage, so people start thinking "Members of outgroup X can't have negative traits". It's best to stay on the object level and prove "This group's average brain volume is 95% of that group's" rather than saying "political correctness", for the same reason proving particular facts is more effective than getting into "What is truth?".

In other cases, like changing the color of garbage bags from black to orange, it's fear of being thought evil and thereby losing status, and attempt to get status by chiding others. This has absolutely nothing to do with the group you're trying to defend, so it leads to white people telling other white people "This is offensive to black people!" while all black people are saying "But we're not offended!" and getting ignored. The phrase "political correctness" used to be useful in those cases.

In yet other cases, shouting "political correctness" is just applying a phrase with negative connotations to people who don't want you to lynch your neighbor.