I wasn't privvy to the original trial, but my understanding of involuntary manslaughter laws is that the prosecution must show that a reasonable person would have been able to foresee that the defendant's actions could cause death in roughly the same fashion that death actually occurred.
I would have argued (I don't know if the defense did) that while a reasonable person would have expected that the fireworks could cause a fire, a reasonable person would not have foreseen that such a fire could within three seconds grow beyond the abilities of fire extinguishers to deal with.
I don't know how Rhode Island's murder/manslaughter laws work, but that sounds about right. Accidentally discharging a gun and having the bullet fly a few blocks away and land on someone is negligent homicide / involuntary manslaughter (whatever your jurisdiction calls it); without getting into quantum of proof, I imagine that the standard for criminal negligence would be a bit higher than for civil negligence. I just can't see someone who designed a faulty gas tank or something being found guilty of negligent homicide if it killed someone.
There was no trial; this was a plea bargain: he agreed to plead guilty to 100 counts of involuntary manslaughter, the DA agreed that his sentence couldn't be higher than 10 years.