It may seem a bit insane but the rapid advance in mining hardware has caused people to be able to mine at insane rates, why is that bad you may wander? Well if you didn't know there are a finite number of coins 21 Million if i'm not mistaken. Also every 210,000 Blocks the reward for each block found is halved, the point is people are mining faster than the demand is being created and if you don't already know when the supply is greater than the demand the value drops, so say we have the max amount of bitcoins (21 Million) but only 18 million are needed thus 3 million are unnecessary (in a way) causing the price to drop
