A lot of cis people react badly to the term cis because they're not used to thinking of themselves as anything but "normal"
nevermind that calling cis people "normal" implies that being cis is the natural/right way of being, and being trans is "unnatural" or "abnormal" - it'd be no different than calling straight people "normal" (which certainly happened before gay people were more visible and the term straight became widespread)
it stems from a lack of respect for trans people, flat out. the people who feel this way think that it doesn't, but it's because they haven't had to grapple with their identity being named before, and having to re-contextualize their experience/identity from being "just normal" makes them uncomfortable and makes them unneccessarily lash out.
nevermind that calling cis people "normal" implies that being cis is the natural/right way of being, and being trans is "unnatural" or "abnormal" - it'd be no different than calling straight people "normal" (which certainly happened before gay people were more visible and the term straight became widespread)
it stems from a lack of respect for trans people, flat out. the people who feel this way think that it doesn't, but it's because they haven't had to grapple with their identity being named before, and having to re-contextualize their experience/identity from being "just normal" makes them uncomfortable and makes them unneccessarily lash out.