Large pressure vessels are essentially bombs; in this case a very large bomb.
Hasn't most of the energy been spent at that point? I was under the impression space launches were miserly with fuel for weight reasons?
You’re right, but it’s still under incredible pressure. You’re talking about liquified cryogenic fuels, so the remainder has greatly expanded over time. Even a small amount of remaining fuel must be vented to render it safe. Volumetric expansion is a bitch!
Fuel in SpaceX rockets (and other working gases) are cryo-cooled, after tanks warm up, even small amount of liquid gas stored in a tank evaporates into a lot of gas which can create very high pressure.
Cryogenic oxygen has something like a 700:1 expansion ratio when it is warmed into a gas.
Take a look at all of the failed landings[1], even after they land on the barge they still go boom(or RUD if you prefer that term) when the structural integrity is lost.