Inverse beta decay does not release energy. It takes an electron, combines it with a proton and forms a neutron, which is more massive than the proton and the electron, so it requires energy to create. My understanding is that under the pressures at neutron star density, it is simply more favorable to occur because it would require MORE energy for the electron to remain outside the nucleus in those conditions. In degenerate material, as the pressure increases the electrons are pushed closer together, but also acquire more energy, as the HUP says that since their position is constrained more and more then their momentum must become more uncertain. At a certain point the energy of the electrons become so great that inverse beta decay starts.
http://en.wikipedia.org/wiki/Degenerate_matter
As far as I know, a quark star would be more dense than a neutron star, but not have an event horizon yet. If it did, then it would be a Black Hole and not what we call a Quark Star.