It's happening because more and more people are buying Bitcoins are fewer are selling. Supply and demand. This interview talks about the real-time increased demand. https://news.ycombinator.com/item?id=5517634
A better way to state it is: "More people are willing to buy at the seller's asking price, than are willing to sell at the buyer's bid."
I suspect pump & dump. Just wait, it will plummet soon when the puppet masters sell their bitcoins.