Ignace Murwanashyaka, the leader of the FDLR rebel group, and his aide Straton Musoni were held on suspicion of crimes against humanity and war crimesJason Stearns, over at Congo Siasa, explains why this matters (incidentally, if you're interested in DRC and you don't read his blog, you should, it's great). In short, while these two leaders have not been in Rwanda for years, the UN group of experts (of which Stearns was a member last year) has documented the FDLR's reliance on its leaders abroad. It's been tricky though to get Germany and other states to act on evidence against these officials. I suspect that Germany in particular might have been encouraged to do so after being briefed by the current group of experts on new evidence on these leaders. As Jason says, you'll have to wait for the report (due out mid-December at the latest) to find out what that evidence is.
Meanwhile, Rwanda has requested the extradition of the arrested leaders. This BBC article also has more on the FDLR.