An Assumption
I had a very minor connection with this incident 12 years ago. But firstly let me say I am not a Pilot and I know absolutely nothing about the Chinook nav system. However, at the time I did have my own guess of what could have happened. (emphasis on MY guess)
You guys obviously know the NAV fit of the Chinook in question, but what I do know is that the aircraft had a GPS fitted. My thought was simply that the crew may have put too much faith in the GPS system and were flying using the GPS nav solution. The actions of the crew indicated that they thought they were further to the west, and most agree it was CFIT.
If there had been a small time error or position error in just one of the satellite signals, then this would have given a small nav error, which the crew would not have detected if cross-referencing to the INS. The error were considering here is commensurate with INS drift if the flight had been airborne 30 minutes or so (can't remember the flight time, but assuming a 0.8 nautical mile system and 30 min flight time)
This would also be virtually undetectable, unless a base station was logging the satellite data of the visible satellites in the region. It also fits in with no technical faults in the aircraft were found.
And if you think that your GPS solution is 100% reliable then maybe today it's close to that figure, but in 94 it certainly was not. If you were to run a base station logging sat data 24/7 during the 90's you would have seen some invalid NAV solutions for some periods of time as the ephemeris data of a satellite degraded with time or was uploaded in error. I remember one day a GPS solution was in error by a few hundred miles for a considerably period of time (maybe 1 hour or more). These occasions were rare, but they happened.