I understand your concern as this has happened to my B three times.
Each time it has been when a possum runs over our flat roof. It seems to trigger his natural hunting senses even when he is in deep sleep. It only has happened when he is deep in sleep.
I spoke to his vet and it was explained to me that it’s a combination of being in a deep sleep and all the muscles, nerves and adrenaline not connecting at the same time creating a shock like reaction. Like fright and flight mid sleep.
But to keep an eye on it.
Ours had always come round after settling with reassurance and calming.
I know this does fix the issue. But it may help a little.
There is nothing worse. It worries me sick too.