What exactly fries the chip when you invert power supply?
Most commercial IC circuits are isolated from the substrate material by a reverse-biased P-N junction (including CMOS parts). The substrate is usually tied to the voltage expected to be most negative.

If it isn't, then that junction becomes forward biased and can conduct a great deal of current, melting metal or heating the junction to the point where it no longer acts as a diode. That is typically at a voltage of about 0.6V, but the IC makers play it safe usually by telling you not to go lower than -0.3V.

(referring to the below diagram, but not shown, the substrate would be tied to pin 5)



Most CMOS parts have another twist that if part of the chip has a normal Vdd and another part sees a big negative current it will trigger a big parasitic SCR that is a side effect of the structure, then the device's power supply draws a large current which causes overheating, melting etc. if the current is not externally limited. That is called latch-up.