Could you elaborate more on what could be common causes of the delayed ignition of the high concentration gas? Hot surface of the vent stack? Oxygen in sprayed water?

There are many potential sources of delayed ignition. Hydrogen is easily ignited, and the larger the cloud, the more likely it is for it to find an ignition source. The cloud itself may serve as an ignition source as the force of the gas release may cause dust or other contaminants to mix in the air creating a static charge which ignites the hydrogen. Similarly, the force of the release can cause surrounding materials and equipment to create an ignition source from impingement. Examples include doors being pushed against surrounding equipment, crushed stone being blown against equipment leading to an impact and spark, etc. The release may also be larger than the hazard assessment or codes anticipated, thereby extending the vapor cloud into areas with other ignition sources such as unclassified electrical equipment, hazardous activity by personnel, or surrounding equipment. 

The oxygen in sprayed water would not be a hazard for delayed ignition