Probably someone who was broadly moderate/centrist and/or who had an erratic pathology to begin with.
Perhaps someone who voted for Reagan because of his promises that the economy would get better, but who was disappointed in Reagan's real or perceived failure to make that happen. Or perhaps the opposite: someone (probably who normally leaned Democrat) who voted for Reagan to "improve the economy," and then by 1984 believed that the economy had recovered enough and that there was therefore "no longer a need" for a Reagan presidency.
Perhaps a Minnesotan who primarily voted for Mondale based on their desire for there to ever be a "Minnesota President" (of course, this person hypothetically would have voted against a "Minnesota Vice President" in 1980, but then again there had already been a Minnesota VP ever in history in the form of Humphrey, meaning that it would therefore be less hypocritical in context).
Or perhaps someone who liked that Mondale had a female running mate, and who voted for Mondale in whole or in part because they wanted to vote for the first female VP.
Possibly someone (most likely R-leaning) with broadly pro-Black political views, who believed that the "final straw" regarding the GOP's relationship with Black America happened sometime during Reagan's first term.
Forgot to mention possibly an R-leaning voter - probably who was LGBT, an illicit drug user (to the extent that any of them lean R), and/or Black, or perhaps who was just a doctor or someone in the medical field - who was dissatisfied with Reagan's handling of the HIV/AIDS epidemic.
