I too think a prop flew off or broke. I can't see the A/C spinning out in the way it appears to have for any other reason. Did you include in your preflight a "tight check", making sure your props were given a snug tightening turn? Did you look your props over for signs of cracks or wear?
If it didn't hit a bird, then a prop failed.
It's funny that you took the time to ask about the one thing that I said I always do.
If I have a preflight check item to test whether the props are "tight enough", why on earth would I have another one to make sure they are "extra tight enough"....lol
Yes. I looked them over. There were no visible signs of damage or wear.
How would a motor malfunction differ from a prop break?
I'm leaning more towards a motor malfunction with my guess mostly because of the pictures which show the props to be stationary and also becuase of the fact that everything recovered and stabilized for a few seconds before failing completely. I can't see how that would happen with a broken or missing prop.