For the first time in two years the difficulty to mine bitcoin dropped on Monday.
Mining difficulty determines how difficult it is to hash a new block and varies based on the amount of computing power used by miners on the bitcoin network. Bitcoin’s growing popularity has attracted more computing power to the network, meaning that the difficulty has been steadily increasing for some time.
This was not unexpected. With the price stagnating recently, and being down over 60% in the past year, the payback just hasn't been there for miners. At the current difficulty many miners are losing money. In order for it to be worth their while, the price of bitcoin needs to go up, the mining difficulty needs to decrease, or the costs of mining (equipment, electricity) need to go down. Something had to give.