One day, someone (prospective client, random jerk, maybe even old college acquintance) makes the mistake of saying something nasty to Foggy. It could be about his appearance, or his lack of finances, or even his sexuality (I personally headcanon Foggy as happily bi, but whatever floats your boat); point is that Foggy just sends back a scathing remark and moves on because he's dealt with this shit before and will do so again. Matt, who is within hearing distance, is not so easily appeased.

Because he can't really go around beating up people without reprisals as Matt Murdock, Matt instead tears them apart verbally for DARING to say something like that to his friend.