I have a Dorset Down ewe who is often found on her back,(my neighbors have got expert in flipping her over). I have seem her sit on her bottom, rub about a bit then gently roll backwards on to her back ....... hilarious to watch but one day she will not be found in time.
She is not overweight and even did it just after she was sheared.
I keep her bum clean, she has somewhere to rub, has no scab, lice or anything, any ideas re stopping her doing it !