Sorry to hear of your loss, especially considering your father built it when he was still with us.

As RCKen says, it's always tough, and disecting the facts afterwards isn't much of a consolation, although it could be argued that it's a sort of "councilling".

It's already been suggested that the Monokote was holed/damaged in some way and the high airspeed you would have achieved in your final manouver could have been what it took to finish the covering off. I would tend to agree with this theory.

In future, it may be worth using either the old fashioned techniques (eg doped on fabric) which to my mind are still the best way, or at least an iron on fabric such as Solartex (which I've found to be tough and durable) instead of plastic film. The reason is that when punctured/ripped, the fabrics are much more resilient to catastrophic failures as you suffered than the plastic films are.

Although it's still possible to tear along the fabric it takes much more force than it does to pull plastic film apart once it gets started.

Once again, sorry to hear of your loss.