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This is typically caused by under watering or sun scald. While it is a cactus, it does need some water and if it has been drier or hotter than usual where you live, you may need to give it some supplemental water.

Sun scald typically happens with plants that are not use to the sun yet, such as a plant you just bought from the store and then set out in your garden. It is the sun burn or the plant world. It can also happen if you suddenly get a hotter, harsher day that you typically would have for that time of year. There is not much you can do for a plant once it has a scald, but they can normally recover from it on their own.

While that should be fine, keep in mind that these plants require very little watering. They are native to arid desert regions and usually have only rainfall to supply their moisture needs. Therefore, only water your barrel cactus once a week in summer to avoid root rot.

Yes, they can be removed and grown. They are called pups. Here is an excellent article on how to remove them and propagate them:
http://rockoakdeer.blogspot.com/2012/05/propagating-barrel-cactus-pups.html

Tehachapi is actually Zone 9 and yes, the cacti and succulents will be fine there. They need to be in well-draining, sandy soil when planted outdoors and should do fine over winter.

Cacti can be infected with fatal fungal rots, and look perfectly normal for months. Then they start to collapse, as if a leak was letting all the air out, and proceed to collapse in a few days. If you had pups already sprouting, you could cut them off and root them, but if they're not already there, it's not likely they'll sprout from diseased tissue. You don't mention if the plant is potted or in the ground, but in either case the most likely cause of the collapse is too much moisture in the soil, or insufficient drainage of the soil. This article might help you with your next attempt: https://www.gardeningknowhow.com/ornamental/cacti-succulents/barrel-cactus/barrel-cactus-care.htm