Mark, I would have to say that the Coaster motors failed at least 30% of the time, especially the "20#" thrust motors. I would bet it had to due with cracks in the BP, most of the time they would just explode at ignition, or sometimes the nozzle would blow out. One of the most dramatic failures was an upper stage that blew out the side of the casing at about 700 feet.......made an interesting smoke trail.
I flew those ProDyn motors too, the long buring E motors in a large styo B/G.... first time it was a little nose heavy and it went up to about 25 feet, then tilted over an went down range with that motor burning like a roman candle.......shedding off parts as it went.......actually glided down about 150 yds away! It was an "exciting flight", a lot of people were ducking on that one...It lived to fly again.
Ned