What determines the ground state of a computers dram?
This is addressed in the paper, in the second paragraph of section 3:


  Over time, charge will leak out of the capacitor, and the
  cell will lose its state or, more precisely, it will decay to its
  ground state, either zero or one depending on whether the
  fixed conductor of the capacitor is hard-wired to ground or
  power.


As charge leaks from the capacitor, the voltage difference across its terminals goes to zero, and the "ground state" of what you can read afterwards is just the voltage on the other end of the capacitor.