There are two things that I can think of that could cause this.
1) a large excess of pressure in the gas main causing regulators to vent and in some cases this vented gas ignited. What could causue this?
2) There was an interruption in gas flow and pilots went out. Then when the gas flow resumed, the pilot supply gas escaped and in some cases ignited. Most appliances has safety systems to prevent this but if the interruption was wide spread, there are bound to be some that don't or that malfunction.
Yes, it is all speculation at this point.