Whenever the price increases rapidly, miners will follow. Huge investors and companies will see an opportunity and will buy miners to mine themselves, many of them not realizing that many others are doing the same and difficulty will increase faster than expected.
it is not exactly unexpected. the difficulty rises to a point where it becomes the same level of profitable as before the price rise. it is always going to be like this. as price goes up it becomes more profitable so more miners come in and increase the difficulty so the profit drops to "normal" levels.
the opposite is also true. if price drops, profit will go down. so miners will leave and difficulty will drop until profit comes back up to "normal" levels.
