CMOS technology is based upon enhancement type mosfets, which r common mosfets with no pre existing channel. A mosfet, or metal oxide semiconductor has a layer of metal (which serves as gate) which is insulated from the semiconductor by a thin layer of silicon dioxide, SiO2 (hence the name, Metal Oxide Semiconductor Field Effect Transistor: MOSFET).
It is the oxide layer that is sensitive to ESD, it cracks, shorting the gate metal with the semiconductor, turning the mosfet into something like a BJT. BJTs r made to handle current while mosfets r not, so excessive current flowing through the device causes it to blow up. (This may not be very accurate an account of what happens but this is the principle behind it all).