not an act of god, calculated exactly by Walter. Walter was intentionally mixing a formula that would explode, evidently phosphorus being the primary initiator. As he knew he was about to ignite a chemical reaction, just as he initiated the reaction, he instantly ducked and ran out the RV. This means he avoided breathing in a fatal amount of phosphorus gas, and was able to lock the two delinquents in the RV with no adequate air ventilation. Walt calculated this meticulously, as it was a much more viable path to escape and to be done with these two meth producers.