Over the past few months, the Bitcoin network became drastically slowed down, due to a very high amount of transactions on the network. This caused other people to look towards altcoins, which offered faster transaction times and lower fees than Bitcoin. Also, lately Ethereum has been doing very well, and this could be attributed towards the hundreds of Initial Coin Offerings (ICO's) that are launching on the Ethereum platform. I hope this explains the graph a little better. There are plenty of other factors at play, too, but I feel that the above mentioned ones are the main ones.