I don't have all the details, of course, but it seems the only thing that has happened so far is that Nagra filed an opposition to Tanagra's trademark application covering its name in connection with CD sales and related business. It has nothing to do with the logos from what I can see.
It is pretty standard practice for trademark holders to file oppositions to applications that might infringe on theirs to test the issue and to prove that they are protecting their rights. Sometimes, the trademark holder is really just trying to make sure the junior user of a potentially similar mark doesn't get rights to use a potentially confusingly similar mark in the same areas of commerce -- in order to prevent confusion and coexist. The applicant doesn't have to do much to respond to the opposition and the trademark office will make a ruling. I'm not sure how the trademark office will react but there is at least a colorable basis for the opposition.
Without all the details, I think that the band's reaction is bizarre and unnecessary. The trademark office action is not decided and they make clear that Nagra had not even sent any kind of cease and desist demand. My cynical lawyer mind wonders whether the band is taking advantage of the situation to get some press, sell some CDs and shwag, and then maybe change their name while they have some notoriety. Regardless of the band's intentions, I don't think this case is similar at this point to Monster Cable vs. Blue Jeans but it could go there if the band doesn't cave and then Nagra overreaches.