Union Berlin made a really good start but they were so inadequate to maintain a good effort later on. They don't have a very impressive squad of course but they had a really good team spirit at first. It is always difficult to resist Bayern Munich's pressure but Union Berlin just gave the lead to them by themselves actually.
Because they started a terrible form and it lasted very long. It was too difficult for them to keep the lead in the meantime. From now on they can at least fight for top 4. It isn't impossible as long as they make the same effort at the beginning of the season.
At least, Union Berlin made a difference even if it was just a few weeks. their work ethic is extraordinary, even with mediocre players. they played their matches with enthusiasm and hard work, and unfortunately their struggle had to be stopped coupled with a fairly busy match schedule. in the end, Union Berlin slipped and had to settle for 5th place in the Bundesliga standings. December is coming to an end, and the Bundesliga will soon be rolling again. I doubt Union Berlin will be able to improve their game again. in fact, to maintain its ranking in the Bundesliga standings must be paid with hard work. IMO, most likely Union Berlin's ranking will be overtaken by Dortmund. in fact, I'm quite optimistic, Dortmund will finish in the top four until the end of this season, provided they don't sell some of their core players.
It isn't easy for this kind of average teams to deal with the domestic league and a much bigger tournament like Bayern Munich in a solid way. I wonder what their secret behind doing this is now. 
