It happened at Fresno Christian High School at about 6:30 p.m. Friday. Dozens of people were in the stands at the time, about 100 feet from the blast.
The explosion was caught on camera by an eyewitness who was driving by. Firefighters are surprised the injuries to the workers were not worse.
"The individuals who were in the trailer were working in the trailer preparing food, and they believe they had a propane leak that they were trying to fix and it found an ignition source and ignited," said Lawrence French with the Fresno Fire Department.
The football game continued once firefighters cleared the scene and made sure it was safe.