It seems fairly obvious to me that methane gas is the cause. How that much methane was allowed to accumulate and concentrate, I don’t know. There is a more modest dairy operation near me where several hundred cows are involved. However, the manure pits are open to outside air to prevent explosions. It doesn’t make for great outside smells at times but it’s a helluva lot better than trying to contain the gas. Perhaps, short-sighted government regulators had the farmer covering his pits to contain the gas for neighborhood ‘environment’ reasons. If so, it will be hard to get to the truth about what may be just another lame-brained governmental-caused disaster.
How was that much methane allowed to accumulate? Because Gaia the Earth Goddess, though the oracle at EPA, was offended by cow farts, methane has to be collected before it can reach Gaia. No doubt there was a handling problem with the methane. But you can be sure modern superstition required it to be accumulated.