My initial surmise is that it was printing too fast. I'd done a max vol speed cal in orca and applied those limits to the filament, but unless I'm missing something, orca doesn't let you set a maximum linear speed for filaments like other slicers do. So when I looked at the print speed and saw that it was much, much higher than it should have been....
Well, I'm going to crank all the speeds down and add a brim and try again.