Supply and Demand.
The increase in mining is what caused this 'cryptocurrency' craze everyone talks about. Effectively "miners" who solve complex algorithms using their graphics card in return for cryptocurrency (Ethereum, Bitcoin etc.). But only powerful enough cards are actually effective enough at mining to actually return a profit.
So in basic terms, all of the miners bought up the middle ranged GPUs that were capable of mining (RX480, GTX 1060 GTX 1070 etc.) but left the low end (not powerful enough) and highest end GPUs (too expensive). This created a huge demand for the mid tier GPUs, which lead to shortages of supply. And the increase in demand along with shortages causes the GPUs price to rise significantly, as people are willing to pay more, to get a good performance GPU.
An example In simpler terms:
GTX 1050 - cheapest, not powerful enough for mining, therefore normal demand, no price change.
GTX 1060 - mid tier, powerful enough for mining, and not too expensive, so EVERYONE buys it and causes a shortage, price goes up to reflect demand
GTX 1080 - high end, too expensive for people to really afford, therefore normal demand, no price change.