It's very much about commercial interests where the National federation want to be able to use the individual player more, which can be problematic due to their club or personal sponsors.
According to the Federation The Union of players gave some new demands shortly before deadlines that they simply couldn't follow, including that the Federation would officially be the employers of the players (which would put extra expences onto the Federation)
The thing is that in Denmark there's a pretty strong labour union making the agreements with the federation. Now that they haven't managed to come to an agreement, the union is asking for solidarity from all players, which would most likely mean that players would have to leave the union if they decide to play, which would not be a popular choice. It would seem like more or less nobody from the Superliga (the top tier league) wants to play meaning that they're now asking the next two tiers.
But it's all pretty complicated. Both sides seem to be throwing mud at eachother making me and quite a lot of other fans feel like it's partially a personal thing between the top people of the federation and the top people of the Union. Personally I wouldn't mind seeing some people leaving the negotiations on both sides (the CEO of the Union has been so for more than all my life!)