Must have been a gearbox warning indication with serious secondary symptoms (whining/moaning/squealing/shaking/temp rise), so the land ASAP cause seems reasonable. Or something like hyd leak detection/isolation in the -60 would make me want to land very asap, because hyd fluid loss can be a killer (but this would be a low probability event outside of battle, i.e. taking a round through the tail). Landing like that for an engine fault or failure is not required - just secure it and fly to an airport with a nice hotel nearby. The other things these helicopters are famous for is spurious engine fire indications (low sun peeking through cowls), but even then, it's easy to look for actual smoke while finding a decent place to land. Doubtful it was an auto - just a split-ass descent and landing. My five cents...