We went to Shetland in 2000 with the Shetland Sheep Society, and toured around some of the time in a couple of coaches. One day everybody spotted a cowped ewe (same as cast) in a very soggy field. The driver was made to slam on the brakes, various sheep breeders in their best high heeled shoes leapt out of the coach and set off up the field to right the ewe - at the last minute she managed to get up herself, and everyone trooped back to the coach, a bit wetter and muddier around the feet
As Graemscifi has said, this can happen when the sheep is in full fleece, especially if the fleece becomes sodden with rain therefore extremely heavy. Our 'bad' neighbour couldn't be ar$£d one year to shear his sheep, so they had two years worth of texel fleece, plus the heavy rain of a wet year, to cart about. They were forever cowping, and Mr F had to jump over fences several times a day to rescue them. You can't leave them as eventually they will die.
It can happen to rather weak, elderly ewes, coming to the end of their lives, when they just don't have the strength to stand up again - time to consider their future at that point.
I currently have a rather chubby Shetland fleece wether who I've seen twice in a couple of days on his back, waving his legs in the air. He's not properly cowped though, because after wriggling around for a while he leaps back to his feet looking very pleased with himself. I think he's just scratching his back, so he'll need to come in tomorrow so we can investigate the reason.
I have had a couple of ewes which cowped - one lost an eye very gruesomely, so I won't repeat that story, the other was heavily pregnant, and had a huge spinning fleece; she had laid down then tipped over sideways into a small depression - no chance of getting back to her feet.
Yes, it's a good idea to hold the sheep once it's standing again to make sure all is back to normal - pee'ed and burped, steady on the feet, before letting it go.