Here's what I can add so far.
These seem to fall pretty much two per pack, placed upside down at the end of the pack. Topps seems to be known for placing inserts and SPs upside-down, so this might be intentional (forgive my skepticism, but we're talking about the set that completely forgot to print a Cavan Biggio card from their checklist...). Users on Reddit are reporting having 22 of these from a blaster, on average, while another user said that every pack in his case had two of them.
All of the wrong-backs seem to be the same across the set. I've read multiple reports of Luis Robert front/Joey Votto back, and Paul Goldschmidt/Mike Trout.
Regular backs do exist for these players, as one Reddit user reported having three of his four Nick Solak cards with the wrong back.
Cardboard Connection doesn't mention these, nor does anywhere else that I've seen yet. However, CC does contain the line "While Topps mentioned "more unannounced surprises," they did confirm the return of Action Images (Hobby only), plus limited Nickname Variations and ultra-rare Throwback Uniforms variations." I can only assume that "unannounced surprises" and "printing a ton of the cards with the wrong back" go hand-in-hand, but it seems that as of now, Topps has been silent.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.