To my speculative thinking there can only be one of three grossly simplistic explanations, each of which ultimately will be explained by many pages of a final accident report:
1. They thought they were somewhere other than where they were;
2. They knew where they were but were unaware of their own true height, or the true height of the obstacle they hit;
3. They knew where they were but something suddenly went seriously wrong with the helicopter requiring an immediately landing.
Experience of a crew doesn't necessarily protect from some types of human factors reasons that lead to accidents.
I do contemplate why all those rescue assets were dispatched in the first place, at night and in bad weather, in response to a fisherman who had, what appears to be, a non-life threatening or otherwise medical non-time critical laceration injury to his thumb.