Couple of other ideas:

Sunbeams are evaporating clouds as well as being cast by a shadow either side? so you get a straight clear patch?

Was there any thunder at the time? High cloud stacks can have 'sprites' at the top during thunder storms which are discharges that jump up into the atmosphere and may leave something behind like that? (curve ball)

I have watched clouds dissappear in seconds when the reach the coast, so little differences in pressure can make a cloud appear or dissappear....

Funny things clouds....

Dave :