For example, say some phrase or usage, call it X, exists. X is used widely, and never with any intention to hurt, marginalize, offend, bother, or in any way cause any trouble. It's been used righteously with pure intentions and loving hearts.
Then a person comes along, and for some reason that nobody can really explain, decides that X is not OK. It is somehow offensive. People don't want to be offensive, so they stop using X. Then as people stop using X, more people stop using X. Eventually only people who are actually insensitive keep using X. Now X is only found mixed in with the rest of the insensitive things insensitive people say, and it picks up a very offensive connotation.
To me, this is all ridiculous and I think the people who look for potentially 'offensive' things, and try to police language as though enforcing softer language will inject love and understanding into people's hearts, are harmful to the world because they distract from things that actually matter to the quality of people's lives and create an easy to point to absurd narrative for the people who are actually acting out of hate to point to, which is an easy way for them to inoculate people into a knee jerk reaction against considering the larger issues involved.