Did you get a chance to save? Or did you die real fast? Any signs of ships on sonar? Did Lurker have a MAD in OM? My guess would be either MAD and airplane depth charges, or homing torpedoes, though I thought you could get away from them by going deep. If you Saved, do a replay and see what you see.
Edit: Alright, after much searching, finally found this one:
Surviving Operation Monsun...