A refinery can leak explosive gasses that form a plume in or around the refinery that rises off the ground. Once detected, there's a fairly high chance that one small spark will set it off but it could be many seconds or minutes until it's ignited and it could take multiple smaller explosions to mix in enough oxygen to cause the big one. This gives the staff "plenty" of time to evacuate but it's still an emergency situation where arguing about carpools or knocking things down frantically looking for keys (causing that spark) will eventually get people killed for no benefit.