Some of the vaulting did not survive (notably, the section that the spire crashed through, and I think one other). The vaulting is what pushes the walls out while the buttresses push them in. (To my understanding, the roof, by comparison was a lot less structural—knowing that fires happen all the time, the 13th century engineers designed a roof that would burn away without destroying the underlying stonework.)
If I am pushing against you pushing the other way, and one of us stops, the other falls forward. That's what happens with a buttress without the roof and internal arches pushing the other way. Not enough to push it instantly over, but give it time or a strong wind...