For the Bundesliga Bayern Munich were still quite dominant in the previous few seasons, even though they recently lifted the trophy in a difficult way. This season the presence of Leverkusen changed everything and Bayern Munich became the team that was not the favorite for the Bundesliga title. Leverkusen's dominance is still so strong that Xabi Alonso's team is unbeaten and they play much more consistently than Bayern Munich as the king of the Bundesliga.
Bayern Munich is not much of a favorite right now because of some of the disappointing results they have had in previous matches. Lazio's aggregate is not big and maybe Bayern Munich can make a comeback in the second leg because the match will take place at Bayern Munich's home ground. Now it depends on Tuchel and how they can get a win in the second leg, as people say that Tuchel is not the right coach to handle Bayern Munich now.
Leverkusen has indeed made a big surprise this season and Xabi Alonso is the main actor, the possibility is quite big for Leverkusen to be able to break Bayern Munich's dominance in the Bundesliga so far, which makes many people happy to see it and it is also for the good of the Bundesliga competition in the future which allows it to be more competitive when there are fierce competitors for the championship title.
Bayern Munich is having a lot of problems this season, and I think it's quite natural for a football team to have problems and Leverkusen has come in the right season to take advantage of it.