Mad Jock perhaps under-states the levels of change necessary but at low level in this terrain the wind is massively powerful and varies from mildly helpful to pretty scary. How applicable that is to this incident no one is clear. Knowing the area and conditions it is unlikely they were low enough for this to be an issue.
But if you do go down to the level of the tops of the high ground life is complicated. Get anywhere near the rotor and all bets are off. So if you have to force land this is a major consideration.
I've picked a good old example of what can happen
http://www.aaib.gov.uk/cms_resources...KG%2008-92.pdfI remember the day as being some somewhat worst than Saturday - if only due to the showers but the wind was also higher and from a different direction. But at that sort of height in Glen Muick you are on the 'shoulder' of the surrounding high ground and running into the lee of Lochnagar - a really bad place to be in terms of rotor and turbulence.
So here was a reasonable light aircraft with two experienced pilots onboard - probably overwhelmed by the weather.