Would closing off the skimmer and taking the sand out of the pool via main drain cause a hairline fracture in the Hydro Air AV150 pump casing and causing the pump to start to leak?

Shutting the skimmer itself would typically not create enough pressure change or stress to crack a properly functioning pump body. It sounds like the circumstances you're describing are consistent with frost damage rather than something caused by the normal operation or adjustment of the pool's plumbing system. When water inside the pump freezes, it expands, and this expansion can exert significant pressure on the pump casing. If the pump isn't properly winterized or protected from freezing temperatures, this can lead to hairline fractures or even more significant damage.