I think it is plain to see that the lost title race last season stung a lot for Dortmunds offiicials. They might not have expected the chance last season but bottling it in that fashion was frustrating nonetheless. I did not think getting Hummels back was realistic but if he is gettable his signing is in line with the more agressive approach that Dortmund has taken.
I believe that the incentive to attack Bayern was always planned for Favres second season, which is historically always his best one, and Hummels would be a clear upgrade to anything we have in defense if he holds his current form, which is the most impressive one in several years. He would also adress several weak spots of the current defense, especially the organisation in open play and set pieces (he has always been a natural organiser and defensive leader) and weaknesses in the air (he is still an absolute beast in this regard).
Him coming back would also signal the end of Diallos short spell with us as he has been linked with Paris for a while now and I don´t see us selling Akanji unless someone offers really stupid money for him. Infact I think that the development of Akanji, Zagadou and Balerdi plays a role here. They could learn a huge deal from a defender as skilled and experienced as Hummels and we have seen with Sanchos development how much of a positive impact the right mentor figure (Reus) can have.
Now on an emotional level I´m pretty torn here. On one hand I view him as a fraud and talker, who profiled himself as Mr. Borussia for years only to make the one move in a pretty pathetic fashion that would enrage the support the most. On the other hand stand his many years of dedicated service to the club eventhough he had lucrative offers for years and I can´t help but feel a begrudging respect for his move back as even if he would have been sorted out at Bayern, he would have more lucrative and comfortable options than facing angry fans in the task of earning back respect from them. I would certainly not doubt his motivation to play for us as the only reason I can see him retuning would be the mission to prove Bayern wrong once again.
Funnily enough, the party I would understand the least in this transfer would be Bayern. I understand their desire to have a squad overhaul, but apart of Süle their situation in the defense is a bit uncertain in this case. Hernandez is a quality defender but has to come back from a serious injury and while Pavard is extremely talented, his last season was pretty mediocre. Selling your best performing defender of the last season half does pose certain risks. In difference to Boateng, Hummels is in terms of profile also a defender who should age better. He is not defined by his athleticism but rather his anticipation, football intelligence and passing abilites, traits that usually don´t decrease much with time. Barring injuries, Hummels could be good example of a defender playing on a high level into his mid 30ies. Maybe they have something in store we don´t know about right now.