Union Berlin this season seem to be suffering a fate that I personally can't tell the exact root cause but then the obvious looks like an effect from the change of coach, they seem to be yet aquinted well enough with this coach and his style so most players are still finding it difficult to cope but then they have still got good quality.
Leverkusen on the other hand continually keeps the maintaining their spot at the top of the league table with their points difference been a bit more that it was from Bayern Munich who happens to be the most fierce contender this season, it's quite interesting that at this point Bayern Munich is yet to be at the top of the table, which indicates that there's possiblity there could be a change in the leagues dominance this season.
I don't understand what you mean, hasn't Union Berlin experienced a complicated season since the start of the season with coach Urs Ficsher? and even though their coach changed just a while ago, I think it's natural that the results are still the same as before because coach Nenad Bjelica also needs progress with the other players so they can adapt to the strategy he gave. It's not easy for Nenad Bjelica because he will be responsible for the results obtained by Union Berlin in the remainder of the season and at least being able to save Union Berlin from the relegation zone is already a good achievement.
The difference in points between Leverkusen and Bayern Munich is only 2 points and in my opinion that is not much because if Leverkusen suffers one defeat then their position will immediately be shifted by Bayern Munich. Leverkusen have been impressive this season and almost no team can stop them.