Weird. Cesium 137 is only produced in spend nuclear fuel as far as I know. Was someone trying to get rid of nuke waste contaminated scrap metal? Soviet maybe?
The poster child for Cs-137 incidents is the Goiânia accident where four people died when a Cs-137 capsule was stolen from an abandoned hospital and sold to a scrapyard. Four people died of radiation poisoning, including a six year old.
My guess is this probably has a similar root cause, someone didn't dispose of a medical Cs-137 source properly and it ended up in the scrap metal stream.