The technical analysis site Fxstreet surmises that the reason for the recent dollar appreciation is that

Investors this week have been covering shorts and closing down losing positions ahead of the New Year. A short position is one which you have sold the asset, and look to profit by buying back the asset at a later date, for a lower price than you sold it. Thin markets and low volumes make balancing books difficult for market players and liquidity providers, and as such wide spreads and enhanced swings in prices can often result. 

It remains to be see in the new year if the dollar can maintain its strength, of if January will see a return to carry trading that will lower the value of the dollar. 

Daily Technical Analysis shows that the depreciation of the Euro has been incredibly orderly, following exactly within the boundaries of a simple channel.