The liner is not intended as a pressure vessel, just an insulator. It can leak around the closures without damage. Grease on the whole liner helps prevent cracking, especially with a loose liner. But, it is ok to leak at static pressure without lots of gas flow and not cause heat transfer into the case.
Where grease is important is at the forward end to help prevent the flow of hot gasses circulating around the closure/case/o-ring joint. The liner alone doesn't prevent this. (Although Aerotech tries using the liner as a seal on the top end with a seal disc, leaking only at the nozzle end). The fwd closure joint also has the highest pressure within the casing. With excess heat softening the case around the forward closure, it will give way there first. For long burns, it's also good to have an insulator on the face of the forward closure (with or without a smoke grain).
Even if you have a good insulation interface there at the liner overlap at the forward closure, it can burn through due to a poor o-ring seal. Incorrectly sized o-ring, too much gap in the case ID vs closure OD, cracked or sheared o-ring, or previously bulging or stress from an earlier overpressure/over-temperature situation (a near cato not realized).
Some clues can be found based on when in the burn the cato happened:
An early overpressure during the burn can split the casing anywhere. Nozzle plugging, giant igniter, erosive burning, soft/spongey grains, uncured grains, etc will cause that. But, there's not enough heat transfer at that point to soften the casing anywhere. It can walk a snapring out, shear a nozzle, shear at a snapring groove, or split open the case.
About 1/3 to 1/2 the expected burn time, that's when an unexpected progressive burn will cause a cato. If the case stays well insulated, this can blow similar to what I described above in the early cato situation. Or, if there's softening of the case due to poor insulation, it will burst there, usually at the forward end (highest pressure point dynamically). The cause of the unexpected progressive burn is usually a lack of grain spacers, where the faces of the grains inhibited each other under acceleration (as I explained in an earlier message in this thread). You'll see this as a normal flame that gets angry really fast! The pressure will go to 2x or 3x the design value.
Anywhere through the burn, a cato could be due to unexpected high burn rate. Depends on the grain design, but 1/4 the way is typical. Propellant cracks or voids, low density foamy grain, propellant not adhering to the casting tubes, or plain old high-g shearing of propellant (especially bigger diameter with large web thickness). The failure will depend on the weak spot in the casing design and how much heat transfer has happened, as with the other circumstances.
So, I guess it's a miracle that any rocket motor works! :wink: