have a feeling that was going faster than it's design limit, maybe "feathering" didn't work?
I remember reading about that when it happened. I think the braking mech failed. There were a few other windmills on the farm and they were all ok. From memory there was like a 1km debris field. Luckily no one was hurt.
You will probably find that it was a test or the machines capabilities and actual destrction limits etc.
Why else would the have a camera mounted on a tripod or something and be filming the unit going at max speed?
Also, most of those generators have automatic feathering systems on them incase they reach overspeed.
Still a good vid tho 
It wasnt a test - the police had cordoned off the area and people were videoing it. Here is a news report about it.
jyllands-posten.dk/uknews/ECE3927005/Minister-demands-explanation-for-windmill-collapse/
I couldnt find the original news story I read on it which went into quite a lot of detail about it. It seems that this particular brand of windmill has had a couple of failures.
