Is the cable you refer to the coaxial with the additional red and black power wires? We used to call it Shotgun Cable and as far as I know the sheath and insulation are both PVC so it should be stable even if it's in wet conditions. Was it just the insulation on the red wire that deteriorated or did the entire cable suffer?

If it was just the power wires that were the problem I'd guess that prolonged current overloading would be high on my list of suspects. I've never seen buried PVC sheathed cables damaged by acidic soil conditions or other underground environmental conditions for that matter.