Burning any chemical with chlorine would be an incredibly ineffective way to produce phosgene gas. Past 200C it’s just going to form chlorine and carbon monoxide, which elemental chlorine in the air is probably going to find some nitrogen to team up with, hence the widely reported pool smell.

Just leaving any chemical with chlorine on the ground is likely to just deep through the ground until it finds a source of sodium and the sodium and chlorine do what they do best.

Between the pool smell, rashes and burns, and dead things in ditch water, and ignoring god know what other chemicals are just in ditch water from field run off, most likely explanation is that some people, and animals, were just exposed to elemental chlorine dissolved in water.