Techland didn't comment on the specifics behind the delay, simply saying “physical production has a longer lead time than digital.”
Fortunately for those looking forward to smashing an innocent undead chap in the face with an electrified baseball bat, the digital release will not be affected.
“There will be a short delay in the release of physical copies of Dying Light in Europe, Middle East, Africa, Asia and Australia,”. “The game will have a global digital release on PlayStation 4, Xbox One and PC of January 27, 2015. As physical production has a longer lead time than digital there is some delay in the aforementioned territories. We will share information on the availability of physical copies as soon as it is confirmed.”
Oddly enough Steama release date of January 30 for the game. Presumably that's an error that Valve will clear up shortly. In any case, while you wait why not check out our to see what we though of the game?