Poor quality control and lack of appropriate testing. Add to that, senior management putting the developers under pressure to release products before they are ready and appropriately tested to maximise potential sales and that's your answer.
Okay, but how much money is this costing the company to take calls, send out replacement units, and deal with the incoming units...all of which could have been avoided? And in this case it wasn’t even a product release, but rather just a software update which could have waited longer.
Apple has REALLY blown it this year:
iOS - buggy and unstable with some things. Mail especially problematic.
IpadOS - same
tvOS - sluggish and had airplay issues
Mac - horrible for some people
And now this. They seriously need to reverse course immediately and start doing things differently because right now they are having software problems across virtually their entire product line with these updates and that is not good. Shouldn’t be happening.