Hummels recently rejoined Dortmund for £34m after three seasons in Bavaria during which he won the Bundesliga three times as well as one DFB Pokal.
Hummels did not play as much as he would have liked last season, making just 21 Bundesliga appearances as Bayern Munich manager Niko Kovac alternated between him and Jerome Boateng to partner Niklas Sule in the centre of defence.
The German champions have reinforced their backline with the acquisitions of Benjamin Pavard and Lucas Hernandez with Kovac planning to switch the latter from left back to the centre of defence.
Kovac reckons Hummels left because he feared Bayern’s new breed of central defenders would pose a threat to his chances of securing a regular starting berth.
"Mats did a great job, especially in the second half of the season," the Bayern head coach said on Tuesday.
"Like [Bayern president] Uli Hoeness said, at the end of the season Mats came to us and asked what the status quo was for the new season.”
"Niklas Sule did a great job, he is a starter of the [Germany] national team and with Lucas we bought a player that we want to play centre-back. That doesn't mean anything.”
"Competition means the best player will play. In that case, Mats wanted to avoid that. We accept that and the result is that he plays for BVB now."
Hummels was part of the Germany side that won the 2014 FIFA World Cup in Brazil, but he was recently ostracized by Joachim Low from the national team alongside Jerome Boateng and Thomas Muller to give chance to youth.
