You probably take for granted, as I have, that "kinda" is a bastardization of "kind of," which you'd use in more formal writing. But could "kind of," instead, be a bastardized attempt to make "kinda" conform to our assumptions? In his OUP "Monthly Gleanings" etymology blog post for this month, Anatoly Liberman points to "kiender" in Dickens's "David Copperfield," uttered by Mr. Peggotty, from Norfolk. Dickens used lots of the dialect he heard around him, and probably spelled it "kiender" rather than "kinder" to make it clear that he was hearing the long i of "kind" and "find," rather than the short i of "kindergarten" or "finger" that we might otherwise assume if we saw "kinder."
Where it came from is much less clear, though it was apparently universal in East Anglia by Dickens's time. Liberman suspects an origin in an old Anglian or Scandinavian word. Don't you just love etymology? I do.