Nobody has the exact explanations, could be just guessing but in this case the high price causes high demand and vice versa. Recently Bitcoin has increased the popularity significantly and more and more users are coming into Bitcoin system. We can also see that more or less all altcoins are getting down which might be the sing that many users are switching majority of funds to Bitcoin.
I totally agree, nobody can actually define specific reasons for this price increase, hence why we can't accurately predict anything regarding Bitcoin's price in the future. It is truly astonishing to see Bitcoin surpass $15.000 today.