I know that KUB is not discussed here in details, but its missile has NO ignitable Sustainer.
Hpasp Free SAM Simulator, "Realistic to the Switch" (U-2 over Sverdlovsk, B-52's over Hanoi, F-4 Phantoms over the Sinai, F-16's and the F-117A Stealth bomber over the Balkans.) http://sites.google.com/site/samsimulator1972/home
This looks like a classic solid fuel rocket booster failure. My guess would be cracking of the fuel because of ageing and/or improper storage. These things are getting pretty old now.