The reason it almost trippled starting from 14 december till now you can find in the Transaction history from bitcoin address: 12sapMKghAPbNDucwAGv3t9fnHQuwzivYP ![]( "Click to send Bitcoin or Gulden to this address")
(details mentioned in this thread on Mar 11, 2017).
That is either Guldentrader, Litebit or Nocks btc address buying Gulden for users who purchase from those sites.
I notice Nocks almost has the same amount of buy support as Bittrex, Nocks will become the biggest exchange for Gulden by end of the month. This is very good for Gulden.
There is a lot more Euro then btc to buy Gulden with and soon people will be able to earn interest with Gulden.