Well there isn't any scientific reports on why cysts forms on T.'s. But hopefully it'll get better within the next molt. It's flipping a coin whether it survives or not, because some T.'s recover from a cyst and others not so lucky. I can't understand why, but IME, I've seen more cyst on the Theraphosa genus than most other genus/species. I hope yours the best on making it out from the cyst.
@Liquifin just makes sense because of their size and humidity requirements. Add springs and isos though. These are dorsal surface cysts so there is a (relatively) good chance of survival as long as there arent more inside. What are you feeding it?