Union Berlin was top of the table at the start of the season. And for a long time, they held the top spot. But suddenly they lost the top spot in the table due to losing points in several matches. But now Munich, Dortmund, and Union Berlin are equal on points. All three teams have an equal chance of going top of the table.
But the way Union Berlin performed against Schalke in the last match, I don't think this team deserves to go top of the table. My guess is we will see Dortmund at the top of the table in the next few matches. Because Bayern Munich's main focus now is on the Champions League. So we will see Munich's unstable performance in the Bundesliga for a few days.
The draw that was obtained by Union Berlin against Schalke made Dortmund move up to second place in the standings with the same points and Dortmund was ahead on goal difference. Even though the domination of the match was successfully held by Union Berlin during the match, they failed to take advantage of opportunities to score goals, because Schalke's defense was so solid and difficult to pass. This draw is also quite detrimental to Union Berlin in maintaining the trend of the title race from Bayern Munich who are at the top of the standings.
I also have confidence that Dortmund will rise to the top of the standings, because in Week 22 Bayern Munich will face Union Berlin who both want to get full points, while Dortmund have a slightly lighter match against Hoffenheim which performed poorly in the previous game. Thus Dortmund's opportunity to be at the top of the standings will be quite open, even if it is assuming the Bayern Munich vs Union Berlin match will end in a draw.