I also don't think it could have been SWP but, I strongly disagree with the statement that you need a rate of descent in excess of 2,500 fpm to encounter one (even in the "hawk"), especially at higher altitudes, my home airport is at 7,340 FT AMSL and you can easily get into a SWP situation with say a 700 fpm true vertical descent (no wind), in any heavy helicopter at this altitudes.
When it comes to SWP its mostly about weight, disk load, and power, and there is not a lot of differece between say a 412 or a UH60, actually the 60 may be even more vulnerable due to higher weight and a lot more power.
I think its irresponsible and ignorant to make such affirmative statements, considering many newer pilots are reading this forums to build up their "decision making criteria" and settling with power is affected by so many variants.
2,500 fpm descent to have SWP, come on. In the position they were a good strong downdraft was all that was needed.
Anyway I personally think it was more like engine related.