Just from an engineering standpoint....pressure tanks are/should be designed to "leak before burst" if they rust or take damage. So that you don't have a catastrophic failure somewhere and get an "explosion" from the energy in the tank that can throw shrapnel etc. So you want the rust to either create a small pin-hole or crack that will leak and relieve pressure, or else some kind of a "blowout panel" that is weaker than everywhere else let go first. Which if yours was a nice regular rectangular piece that popped out, sounds like what it might have been.
So it may well have done exactly what it was supposed to do under the circumstances. Good to know. :) But all good advice about draining the water out.
Just some random thoughts from an engineer. :)