With absolutely no background in Sea Kings or accident investigation, but a couple of glasses of wine to sharpen my ESP...
Tail rotor driveshaft or txmsn failure (clean driveshaft break, oil out of T/R housing vent), right yaw develops, pilot puts it DOWN. Note position of tail wheel, pylon, and relative lack of damage to T/R blades (considering they are holding the pylon up).
Good on 'em for getting it down with nobody hurt, and someday we'll know what actually happened.