Ouch, exactly why people are told not to leave batteries charging unattended. Good thing you were close to hear it.
But not "too close".
Really, it doesn't help to be nearby when these things happen. They can richochet all over the place.
Attended charge, i.e. mindful that you have something on the charger, out in garage or in a lipo bag or metal box (I used an old milkbox that they delivered milk in in the old days that I picked up in a junk shop) similar to Bowdens "money box" in previous photo.....with holes drilled in for cords.
I have never left my batts or charger out in the open in my "living space" . Seems like a very bad idea! Even with best equipment