He asked a question at a press conference to the effect of "there are a lot of rumours you are corrupt, do you have any comment?" - but his motivation to ask that question wasn't that he expected a good answer. Rather, he was advertising his interest in the story to anyone who had info they'd like to leak. And just like that, he starts getting tips and leaked documents.
Süddeutsche Zeitung is in a similar position. By handling previous leaks well they've advertised several things: (a) They have the knowledge to recognise the importance of such documents, (b) they consider such content within the remit of their paper, (c) they won't bury the story out of fear of the rich and powerful (or on the advice of a risk-averse legal team) and (d) they can do an adequate job of OpSec and source protection.
For a leaker, the choice between Süddeutsche Zeitung and Teen Vogue is obvious.