

some foods (and other things, like blood and some other medical products) are irradiated in order to sterilize them and make them last longer, 137Cs sources are used for this purpose because this material is easily available but can’t be used for other purposes (like radiography)
the source getting damaged or corroded somehow is the simplest explanation i can think of now, cesium is likely in form of chloride which is very easily soluble in water
or maybe it was intentional sabotage by competitor, who the fuck knows
https://en.m.wikipedia.org/wiki/Tetramethylenedisulfotetramine