Price drives difficulty. Not the other way around. That's the way it is.
Sorry, this is wrong. Actually, thinking of this sort doesn't even relate enough to reality to be merely wrong.
The exchange rate does not drive difficulty. However, the difficulty also does not drive price.
Price, difficulty and a bunch of other factors form a complicated system of non-linear differential equations. In short, they all drive each other in a chaotic way. And I'm using chaotic in the technical sense here, meaning deterministic but unpredictable, like the weather.
A quick glance at a difficulty chart and an exchange price chart should make it perfectly obvious that there is no simple relationship between the two, so I can't imagine why people keep insisting that one causes the other.
References that you should probably read before replying:Chaos theoryNewton's Method
for finding roots of equations. A ridiculously simple system that is often totally unpredictable.Lotka Volterra
a very simple model with only two equations. Pay attention when you get to the part about the atto-fox.