The Europa League final was really unexpected. At the beginning of that season, we considered Liverpool as the favorite team and we thought that Liverpool would qualify for the final, but Liverpool was eliminated long before the Europa League competition. Then when we saw Leverkusen's consistent performance in that season of the Europa League, we had a lot of confidence in Leverkusen and Leverkusen qualified for the Europa League final. Leverkusen's opponent in the final was Atalanta, Leverkusen had performed much better than Atalanta that season, but in the final match they defeated Leverkusen by a score of 3-0. In final matches, nothing can be said for sure because even bad teams sometimes perform well in the final match.
It seems you're talking about a Europa League that didn't happen last season, because that season's Europa League final was contested by two English clubs: Manchester United and Tottenham Hotspur. What you're talking about was a season when Leverkusen was in great form under Xabi Alonso, although they could still lose in the final against Atalanta, as Leverkusen had never lost a single match before, including in the Bundesliga.