I guess it really depends on the propellant. What the hell was he doing?
Well, the article does
say "...laboratory that researches high-risk chemical procedures". So, I would guess that he was trying a process known to be potentially deadly, and oops, mixed too much. There are a lot of compounds in this class. I know of a lot of chemicals used in industry that will kill you in a heartbeat, just by poisoning, they don't even have to explode.