In English, I can easily come up with many words that might be used; given that the usual reaction is "Oh my god, are you all right" followed by "What the ***** is the MATTER with you???" or something to that effect, I was looking for the right Japanese swear word to use. The person in question reverts to Japanese for curses and such, even though most of the time they speak English.
I thought of using "baka", but that seems awfully mild for the circumstance. "Bakayarou", on the other hand, often carries an implication of hostility towards the person themselves, as others have pointed out, so I'm not sure if that's the right word or phrase to use either.