Ok, help me out here.
Isn't the problem due to the black powder propellant cracking and this is far more likely
on the larger sizes?
Is this a problem with composite propellant?
Cracked BP grains is almost NEVER the problem. It was the problem for select motors or batches, such as the FSI E60, F100 and the Year X Estes C5 motors. The C5 had cracks originate in the centerbore after they cured ("cured" = dry out since they are pressed with damp propellant).
Most catos of motors, such as the long ago D12 issues of the 1970s and the more recent E9 issues are a simple weakening of the mechanical bond of the propellant to the casing wall. If the nozzle pops, and the propellant stay inside, that is the nozzle to casing bond failing. If the nozzle bond is normal, when the flame front reaches the wall of the casing, it can propagate up the sides of the propellant grain - resulting in a HUGE overpressure. This will blow the grain out the top like a Roman Candle. It can also blow the nozzle as the pressure gets very high. IF the delay train is bonded really good, and the nozzle stays bonded good, then the excess pressure has nowhere to go and the casing splits (pretty loud). That would happen with some of the Estes BP E15 motors and some of the Year X C5 motors.
Now, how does the flame get to the propellant wall "too soon" after ignition and what makes the propellant to casing bond fail?
Temperature Cycling. The cardboard and propellant expand when they get hot (sitting in a hot garage/car/USPS truck/etc.). The propellant is nice and solid and it expands in length and diameter. It stretches the cardboard casing which is made of wood fibers with some glue. When the motor cools, the propellant returns to it's original size, but the casing may not shrink back at the same rate or it may stay slightly stretched. This can compromise the propellant to casing mechanical bond, which allows the flame to propagate up the inside.
The nozzle to propellant interface also expands at different rates, so there is a small crack that forms around the outside of the nozzle to propellant interface. This allows the flame to reach the wall shortly after ignition and before it normally would get there (at the time of peak thrust at the top of the thrust-time curve).
I tested hundreds of D12-5 motors to help confirm temperature cycling was an issue and to help define the exact failure mode.
To crack the grain, you need to lay the motor on a concrete floor and hit the motor with a large hammer.
From what I've heard, Estes worked hard to get their casing suppliers to produce the highest quality casings to meet their specifications to help reduce/eliminate most of the problems seen in the D12 in the 1970s.
I suspect that most of the 'spit nozzle' failures of the current generation of motors are from the clay material properties. The current clay is what I call "Strong Like Ox". it is WAY better than the "Poopy Clay" used back around the release of Star Wars Episode 1 and the mass of rockets and motors produced at the time. That "Poopy Clay" was not good.