The only concrete thing I've seen is the information on Boundless Informant.
http://www.guardian.co.uk/world/2013/jun/08/nsa-boundless-in... Which shows that Germany was a rather large target of NSA surveillance.
The rest is pure speculation. First, is NSA surveillance in Germany actually disproportionate or did the leaked slides not account for population differences. Maybe, but according to the slides, there are on the order of as many intercepts in China as Germany. So lets assume its not erroneous.
Why does the NSA have a disproportionate interest in Germany? Are they of massively more geopolitical significance than their neighbors or does the USA have a dearth of insight into their foreign policy thinking via back channels? I'd say thats more likely to describe France with its Security Council seat and less strong working relationship with the US.
Is the NSA engaged in massive economic espionage? Possible, but I can't imagine that accounts for enough as that would likely be rather targeted.
So the last reason I can think of is counter-terrorism (one of the NSA's main focuses). Mohammed Atta was part of a cell in Hamburg after all. Ok, but why Germany and not say Denmark or France which both have similar populations you might expect(rightly or wrongly) to be under similar surveillance? As someone pointed out on HN a while ago, Denmark does extensive surveillance and telephone monitoring themselves. I'd guess France does too. Does Germany? I honestly don't know, but given the German's focus on privacy and the legacy of the Stasi, I certainly could see it being a problem and being easier to outsource to the NSA.