I believe it started about 13, 14 years ago, a late night show host (was it Gotschalk?) said Oliver Kahn looks like a chimp and walks like a chimp. From then on, in every stadium he's visited, opponent fans just threw bananas at him, and imitating chimp roaring to him. Maybe the refs are buying that as well, they are going to tolerate a little bit more if he acts like a chimp as well.
Just my wild guess.
Just my wild guess.