As far as I'm aware it's the fact that you're going very fast, combined with very high loads on the rotors, which causes them to collide.
At low speeds, both cone by similar amounts, but at high speed, the retreating blade 'bites' the air less (because its speed through the air is less - the blade's speed minus your forward speed) meaning the rotor cones more on one side. Because the two rotors spin in opposite directions, you get the high cone on opposite sides for each rotor, so eventually they touch, with appropriately disatrous consequences.
Knowing that - slow down or don't turn so hard and you should stop it happening.