As we have already mentioned doping level of Zener diode is very high and hence width of depletion region is less. As we know
E = VB / d
VB is the barrier voltage
E is the electric field
d is the depletion width
As doping is high, hence width (d) is less and as barrier voltage varies with doping as stated by the formula:
From the formula we can get that the voltage varies proportional to log of doping and hence the barrier voltage is almost constant.
So from the above discuss we find that Electric field in the depletion region would be large as VB is almost constant and d has decreased. Due to this large electric field, electrons from the outer shell of the atom in the depletion region are expelled out and hence carriers are generated within the depletion region. The high electric field in the depletion region pulls out large number of electrons from the large number of atoms. This leads to large current flow and this type of breakdown is called Zener breakdown.