A: How are you?
B: I am mad.
A: How come?
I thought that how come was a logical word choice but upon speaking with somebody for whom English is a second language, I found that they did not understand the meaning. I am wondering if using how come in this manner is a form of slang.
For some reason saying "How come you are angry?" sounds like bad grammar.
I can imagine the use of "How come, when I talk to you, you are angry?" to be correct with that added phrase in the middle.
So my question is, is using how come as in the example conversational slang? And also is "How come you are angry?" proper English?