when i break down all the upsets from major European leagues of 2010-now, i have to say that the Dortmund one is the least impressive to me. Monaco (95pts, PSG the seasons before and after - 96pts, 93pts) and Atletico (90pts, and though its rare for both barcelona and Real to finish bellow 90pts in the same season in the Ronaldo&Messi era, and most of the times both have 90+pts in the same season. this was in fact the only one instance when both didnt reach 90pts, but both still had 87pts) were very impressive, especially when both made deep CL runs. Leicester was unbelievable, but 2nd place Arsenal had 71pts. the very next year 5th place Arsenal finishes with 75pts, and the year before 3rd place Arsenal with 75pts. as much as it was Leicester, it was also the entire big 5-6 choking to legendary levels, all in the same season.
now for Dortmund
most people will see that they beat Bayern to the title 2 years in a row, but you need to take a deeper look at the situation in the BL in previous seasons all the way to the Bayern treble. after losing to Wolfsburg in 2009 (69pts vs 67pts), they signed Van Gaal, Robben, Gomez and Olic. the next season they were a similarly mediocre team that almost won a treble, thanks to Van Gaal installing them an identity and Robben carrying the whole team on his back. won the BL with 70pts. the other great thing Van Gaal did was introducing their talented youngsters to the 1st team, like Alaba, Badstuber, Kroos, Muller, so naturally, as they grow and the team improves, next season you would expect even better results (in the BL mainly, hard to improve in the CL after just reaching a final). but next season was a major disappointment. Van Gaal and Hoeness are both stubborn mules, so as soon as things didnt go well for Bayern, it was bound to bang up. Van Gaal almost cost them the CL qualification (to Hannover lol), but they fired him just in time before it was too late. Dortmund (a team that lost in the DFB Pokal to a 3rd league team and went out in the EL group stages) won the league on 75pts, Bayern was 3rd on 65pts, barely qualifying for CL after Hannover choked away 6pts against 2 relegation battlers. they let Van Bommel go midway through that season, who helped Milan win their only scudetto in the last 14 years, so that didnt help either. they really had 4 great players, Robbery, Lahm&Schweinsteiger. the rest were either mediocre, past it or up and coming but still not ready. Robben played in only 14 games that year as well. now youd expect Bayern to load up after a lost title, like they did in previous failures, but they only added Boateng and Neuer and brought Heynckes back. Dortmund lost BL player of the year Sahin to Real, replaced him with Gundogan, and added Perisic for depth. then they beat Bayern for Reus in the January window. sidenote: id consider Napolis run from last season the more impressive one despite not winning the league, since they pushed us to win 95pts to win the league.
the 2nd season was more impressive for them. Bayern had a better year, they were leaders at the winter break, Dortmund was out of CL in the group stages (dead last behind Arsenal, Marseille, Olympiacos), and it looked like its back to the old Bayern "dominance". Bayern played in a CL final that season, but had a mediocre 2nd half of the season domestically (lack of depth might have taken a toll on them). finished 2nd with 73pts, Dortmund 1st with 81pts, winning both games of the head to head (considering they won the league by 8pts, this was decisive), and beating Bayern in the DFB Pokal 5-2. this feat is still Klopps most impressive one of his career together with beating Real in the CL semifinal the year after, and it happened 6 seasons ago. plus he did have a great team as well, Weidenfeller, Pisczek, Subotic, Hummels, Schmelzer, Kehl, Gundogan, Gotze, Grosskreutz/Perisic, Kagawa, Lewandowski. the very next year Bayern won 91pts so they left them no chance, but Dortmund still finished with only 66pts, 15pts less than the previous season. i guess when you too have to play the CL, it does affect your team after all huh.
-1st title was Bayern barely qualifying for CL, so they had little competition
-2nd title was the more impressive one due to Bayern staying in the race until the end, but not as impressive as Monaco and Atletico since they did it on 2-3 fronts.
-Klopps Dortmund was still a great team, he wasnt coaching a bunch of no names