Just because things are correlated doesn't mean they aren't causing each other.
Sometimes when two things appear to be linked, they really are. Throwing out a bunch of examples of things that aren't isn't helpful at all.
Anyone who has thought about mining should realize there is a very clear and obvious reason why the two are linked. The first thing I do before buying mining hardware is attempt to estimate the time to payoff. If it's longer than 6 month, it's probably not worth buying the hardware. So, if the difficulty drops in relation to price, it makes a lot more sense to buy hardware and mine, thus pushing difficulty up. If price drops while difficulty remains high, or if difficulty increases and price does not, it makes more sense to BUY bitcoin outright and forget about mining. As a result, the value of bitcoin and cost of mining (directly associated with difficulty) correlate fairly well over the long term.
And the this would be typical calcs I would want to do... Xx70 prices vs. Price, FPGA price vs price, difficulty vs. Price.
And then search for data points that do NOT fit such models. Fun stuff for the free time of a novice researcher, if you ask me.