I wonder what was the final piece in the puzzle. What or who gave way/agreed to clear the way for this.
This is just my assumption: Market pressure. I assume that Apple is now prepared to scrap bringing Girocard (the German debit card system) into Apple Pay and waiting for major players such as Sparkasse or Volksbank. Google Pay and the many new apps being launched on Android by the banks have put mobile payment into the limelight. It looks bad when all the PR states "but Apple users aren't able to pay with their iPhones".
My assumption, based on scraped together pieces of information is a launch similar to that of Google Pay. 1-2 big banks (I would put Commerzbank in that field), 2-3 smaller players (I think N26 and boon are safe bets). Anything more than that (such as ING / DKB / bunq / ...) would be a bonus. I hope that some banks realise, that you can also offer a MasterCard or VISA debit card with similar acceptance to Girocard. I would use one of those straight away (such as Commerzbank, they offer that in tandem with a Girocard).
Then try to do what happened in France: Bring in more partners as time goes on. Apple Pay is something for the long run. It doesn't need to be a big hit right off the bat. Apple knows that.