[jojobas]

>then expands rapidly

Except it doesn't. The breakup has roughly the same dynamics as dirt spreading as it gets out of a tip truck. The shockwave is not produced by the outward motion, it barely has any speed.

[margalabargala]

You're conflating the outward motion of the rock with the outward motion of the air.

The shockwave is traveling at the speed of sound.