I've read enough propaganda where I can figure it out. Probably started off with Smalcald and went downhill from there. I can imagine, though, that the Tudors might have felt somewhat threatened by them, as well (thus causing a lot of the English anti-Catholicism...from whence ours originated, I'm sure).
(Please to note, this is not to say that Catholics were all wonderful and without sin during those years either...but that wasn't the question)