Manure pits, or deep beds of animal litter, can generate explosive quantities of methane. But the usual result is not an explosion, but a mass death of the animals in confinement, asphyxiated by the ammonia and methane fumes that build up.
Not so much that the facility was vapor tight, as the ventilation system (usually large exhaust fans) failed in some way, allowing the accumulation of the fumes.
Manure pits, or deep beds of animal litter
That would seem a strange condition for inside the barn. Typically a barn is going to have humans working inside from time to time.