This is when one of your mates has a special ability to "go off" on a mad one and get overly angry or physically violent within a matter of seconds; amongst a group of people participating in an otherwise "Socially Healthy" environment.
For example, maybe a barmaid getting he/she the wrong drink...or having a pop at the "no spray no lay" guy/women in the toilets in Yates (High Wycombe).
Or perhaps, when at an afterparty at somebody's house, he/she decides to cuss someone their hardest down the phone, in front of people he/she has yet to make a "good 1st impression" with....and that you have yet to introduce.
You would expect to use this expression like this:
You: "Sorry about them..he/she is a social hand grenade. Most of the time in public, he/she is fine but it only takes someone to pull the pin and thats it...he/she will go off in your face!"