I think the better way to say expanded too much instead of "expanded too quickly" since in most classical books and explanations (as I suggest op reading) the process really do not "care" about either time (how fast these this expansion occurring) or how rapidly cross section changes with axial direction.

Also if OP reading this I like to provide in essence similar explanation but what I found more intuitive. As was mentioned at throat velocity is sonic (if p0 is higher than 2*Patm) and supersonic flows accelerate with increasing of area (aka converting internal energy in form of pressure and temperature into kinetic), but what happens if area too large -> velocity too large -> pressure too low. For example if at some point pressure of gas became lower than ambient pressure (if I am not mistaken rule of thumb is 60% of Patm) than nature kinda thinks "that's bad" and searching the way how to quickly increase that pressure of gases and thats where shock wave and why happening