Filled and slightly pressurized ball with flammable gas + oxygen/air mixture. The water cooled the ball causing the pressure to drop which allowed the flame to enter the ball causing boom.

Mistake was filling ball with fuel + oxygen… pure fuel wouldn’t have created that nice of a flame but wouldn’t have been able to explode like that.

Source: I was a kid who had unsupervised access to a welding shop while growing up. Oxy-acetylene makes one hell of a bang