To be honest, many of the sheep around here are casting their fleeces - I just managed to get my Shetlands sheared before one or two of them cast their fleeces entirely.
If the skin underneath looks fine, the new wool is growing back well and she seems happy, it may just be the normal rise which has allowed her to cast her fleece.
Sheep can have a break in their fleeces because of stress, parasites or illness, so that needs checking for.
But the ability to shed their fleeces which has been selected for in Wiltshire Horns, came from somewhere - the genes to do it are in the general sheep flock, wild sheep did it - and some individuals do it more than others.