Some how I felt like there were some level of pressure around the game that Leverkusen couldn't handle because at this point everyone was aware they were about to make history and all that meanwhile Atalanta knowing all this understood the pressure and how well to take advantage of it to seeing how well they can be of a problem to Leverkusen and stop them from been able to win the trophy already so they added more pressure and increased it for Leverkusen to been able to handle. An early goal has always been the tactics of Atlanta as they did used it against Liverpool and it did worked.
Leverkusen would have been able to avert that losses if they were able to calm the pressure at the beginning of the game as that has already gotten them to have some flaws already in their form and their morales were down when they were now two goal behind because in oder to win they have to cancelled the two goals first after which they can now hop to score more goals to win.