Dortmund fans have a unique way of protesting ticket prices. During the match against VfB Stuttgart, fans arbitrarily threw tennis balls on the pitch. It was oddly beautiful, especially in slow motion:
According to The 42, much of the animosity is over the prices away fans have to pay:
The attendances in general have been down at Stuttgart this season partially as a result of high ticket prices.
Dortmund supporters are being charged €19.50 for standing tickets and €38.50 for the cheapest seats, while a quarter of the tickets being charged to the away fans are as high as €70 not including advance booking and postage fees (€1 and €6 respectively).
This is kind of like if Kevin McCallister from Home Alone became a soccer fan.