What causes it? It feels like a burn, and it can persist a few days.
I think it really is a burn, but in the acid sense of burn rather than the heat sense. They’re full of citric acid and if you eat them by letting them dissolve on your tongue, the localized concentration of acid can be pretty high.
They’re extremely acidic, and also basically turn into sandpaper as you suck on them.
because the tartness comes from citric, tartaric, and malic acid. back in my dumber days I’d eaten whole rolls of Shock Tarts only to have my tongue feel like I slurped from a car battery.
I vote for the sandpaper effect. It always gets me.
When I was a kid, we used to get giant SweeTarts that were the size of hockey pucks. Basically, all you could do was lick them – they were way too big to bite. More like a belt sander than sandpaper. I think I remember my tongue bleeding.
Ditto that.
Damn I miss those, right down to the bleeding tongue. The big ones available now are soft & chewy (and also only about half the size).
Had to laugh at this one - over here a “sweet tart” would be a pretty young lady with loose morals.
Ah, pretty young lady with lose morals. 
I remember those! (With a mix of pain and pleasure…)
My mother always called them “acidy junk”, and there were/are so acidic.
And delicious.
Edited to add, though you couldn’t really bite them, but you could slurp and gnaw at them.
Ah, memories.