A Zener breakdown is actually a quantum mechanics function that has electrons tunneling through the depletion region. For those of you not familiar with quantum mechanics tunneling, in short, it's where an electron overcomes an energy barrier higher than the energy level it currently has. It shoots right through, despite the fact that it seems impossible. To encourage the statistical likelihood of this quantum tunneling (there's something about the word "quantum" that makes a person feel smart), Zener diodes are heavily doped to decrease the size of the depletion region, giving them less width for the electrons to have to tunnel through. This typically happens at lower reverse biased voltages.
The lower voltage breakdown in a zener diode.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd