"Words the writer wishes to emphasize in a sentence are also sometimes written in katakana, mirroring the European usage of italics." http://en.wikipedia.org/wiki/Katakana
Furthermore, the word baka in particular is often written in katakana without much of any reason. Both the hiragana and katakana versions show up in dictionaries: http://jisho.org/words?jap=baka&eng=&dict=edict
