Too steep a turn at too low an altitude. When the rotor disk is 90 degrees to the ground, there's zero lift. There's a vid of a similar crash involving a B-52 that turned too steep and too close to the ground. Just the loss of a few feet of altitude is enough to put you in a non-recoverable situation.
Hopefully the pilot wasn't badly hurt (or hurt at all). It was a slow-speed glancing impact, so it was probably survivable.
The driver certainly looked and sounded horrified. Top Gear is risky business. Just ask Hammond.