If you are into generalities, then you can say that anytime a helicopter does something unexpected or uncommanded, barring a mechanical failure, it is pilot error. An error being any time there is a deviation from optimum. An altitude deviation is an error, a 1700' altitude deviation is a larger error.
It would seem impossible to lose yaw control in an R44 as long as you have full RRPM, but it happens. Pilot error? Sure, because what else could it be. I watched a hovering Raven II get spun 270 degrees by LLWS - in this case it was the pilot's judgement error for being in the air just as the gust front arrived (he was trying to get the ship back to the hangar before the storm arrived). Luckily, he reacted properly to the situation his initial error caused, and no injury or damage done.
Most of my flying is a series of corrected errors - the trick is notice the errors when they are tiny, and correct them rapidly (preferably before they become noticable to anyone else).
So absolutely, if this R44 didn't experience mechanical difficulty (and apparently it didn't), then the pilot made a mistake of judgement, action, or reaction - most likely all three. Exprience is a tough tutor. Test first, lesson after.