I think it depends on whether or not the button-pusher is really deliberately trying to hurt and offend the person in question. If they are, then I think other people should jump in and basically give a "Hey, that's not cool, you need to be back off!" If they don't know, then I'm sorry, but I think it's up to the sensitive person to either back away from the situation or control themself. Absolutely they can later explain they are really set off by joking about X topic. Or if they can't bring themself to talk about it, then they could get someone else to explain it to them.
I don't believe that a physical response to having your berserk button pushed is truly uncontrollable in most cases. When I hear about these things happening, often there's a tone of 'and good for him!', because these stories are typically about someone 'going off' on someone of somewhat equal physical capabilities. Would the person who 'went off' have been able to control themself if the person who'd pushed their button had been a six year old boy or an 80 year old woman? If so, then there was a choice in what they did, it wasn't an involuntary response. Same as how most of these people would probably not go rage-punching 12 trained martial arts experts.
I am not talking about people here who have something like PTSD set off, but just people who 'just get so angry' at an insult they need to respond violently.