there was an incident
Yes, it was widely discussed in Polish media, and the consensus that was reached by the experts was that the incident had nothing to do with any anti-German sentiments but rather with phonological effect that the German language, spoken with Polish accent
, has on people. It is often claimed, all around the world, that the German language, in its spoken form, lacks any intrinsic and profound beauty. Whether it is true or not is, of course, debatable. However, no one in his/her right mind can deny that German spoken with Polish accent is a phonological atrocity.
a Polish university professor was head butted for speaking German
Let me be clear on this matter: I certainly do not, and I couldn't stress it more firmly, condone such behaviour. It would, after all, seem somewhat uncivil to headbutt a complete stranger on a public tram (whether said stranger is a university professor or a representative of any other profession). But I do understand what caused the lamentable incident.
Would I, personally, headbutt a Polish person for speaking German? No... I like to think that I wouldn't
... but my honesty forces me to admit that I'm not 100% sure.