Who was the one sabotaging the ritual and why?
Marty was sabotaging it! Everyone else thought he was dead, but in fact he had discovered the elevator into the Facility and was futzing with the wiring, trying to give his friends time to escape.
He explains this to Jules very briefly as they are getting into the elevator. The concerned look on Sitterson's face is meant as foreshadowing of the surprise reveal that Marty is not quite dead.
(It took me several viewings to figure this out.)
Having checked online scripts, I realize now the sequence is much subtler than I remembered. Sitterson is discussing it with some guys from Demolition, one says, "That [the problem with the fuse] wasn't our fault. There was a glitch. Power re-route from upstairs."
Sitterson responds with "What do you mean, upstairs?" (Upstairs is the cabin and the subject of the ritual, not senior management.)
At that moment, the hot-line rings and we hear Sitterson fielding an unpleasant call from the Director: "That's impossible. Everything was done within the guidelines. The virgin's the only one.. No, I am not doubting you. Which one?"
Then there's a match-cut to Dana exclaiming "Marty." Marty is alive and the implication is, as Sitterson feared, he caused the glitch.
At the elevator, he tells her, "I've been playing around. I think I can make it go down."
I conclude from that that by "playing around", Marty deliberately or accidentally caused the glitch that almost allowed the other three to escape, but it's possible I am overthinking it.