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

Spraying water onto a vent stack, either for gaseous hydrogen or liquid hydrogen, is not recommended. While this is prohibited within the code for liquid hydrogen due to the much greater hazard of plugging the vent system, it also presents hazards for gaseous vents as well. The water can enter the vent system and plug due to ambient conditions. In addition, if the water was sufficient to extinguish the fire before the hydrogen flow is stopped, then a flammable or explosive cloud may form which can reignite unexpectedly leading to a greater hazard. 

Water is only recommended to cool equipment adjacent to a hydrogen fire.

NFPA 2 provides Tables in Chapters 7 and 8 that specify the hazardous area classifications surrounding vent stack outlets. These are based on typical vent systems and flows, but are only applicable for smaller systems. The designer of a vent system should apply the principles of documents such as IEC 60079-10-1 (also required by NFPA 2) or NFPA 497 to evaluate larger vent releases where the hydrogen cloud will extend beyond the prescriptive distances.

Delayed ignition is a significant hazard for hydrogen releases, either intended or unintended. The primary concern is the overpressure and energy release created from a vapor cloud which could contain a significant quantity of hydrogen. The H2Tools Incidents database contains a number of examples of delayed ignition. 

It depends on the construction and location of the lights. Assuming the typical practice of using non-classified lighting on vehicles, operation of those lights during a delivery could provide an ignition source if located within the classified area near the delivery or venting activity. A more modern LED lighting systems may present a lower risk.