If the facts of the incident are accurate, I'd say that using either term "spontaneous discharge" or "accidental discharge" would be correct.
"Spontaneous discharge" would definitely not be correct. Anything spontaneous happens without outside initiation. For instance, barns have burned down due to spontaneous combustion of the hay stored there. Especially if the hay is not completely cured when put in the barn, as it continues to cure it generates heat. That heat can build up inside the hay to set in on fire. When I was a kid growing up on a farm, a neighbor's barn burnt just that way.
I am not even sure I would call that an accidental discharge. Definitely an unintended discharge.
(Luke 11:21 KJV) When a strong man armed keepeth his palace, his goods are in peace:
(Luke 22:36 KJV) Then said he unto them, But now, he that hath a purse, let him take it, and likewise his scrip: and he that hath no sword, let him sell his garment, and buy one.