Not sure. I think it's a combination of people getting greedy and the fact that the segwit activation date is becoming closer and closer instead of something that we need to speculate on. It's pretty much going to get activated by the end of the month, guaranteed.
However, most of the reason for the pump is probably people just thinking that bitcoin is a get rich quick scheme, and just going crazy not thinking about making the right decisions. I would definitely not buy in at the moment even though it may well go up in price further. It's not a risk that i would be able to take comfortably.
the answer is one word:
demandthink about it. price is now $4200, people are now paying forty two hundred dollars to own 1 bitcoin. they want it. the media is helping a lot too. it is no longer a secret thing existing in the nerds basement in 2009

it is out there, countries are seeing it as a currency just like USD, CNY, JPY, GBP,... and that is a huge step forwards.
Of course it's demand, however, most of its demand is coming from speculators instead of average Joes who want to have a stake in bitcoin. These speculators are going to pump the bitcoin price into an artificially high level. Not sure if this is really good news, or not.