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
The best known Chinese rodenticide, containing about 6–20% TETS, is Dushuqiang, "very strong rat poison". It has been used for mass poisonings in China: in April 2004, there were 74 casualties after eating scallion-flavored pancakes tainted by their vendor's competitor; and in September 2002, 400 people were poisoned and 38 died from contaminated food.[11][12] In 2002, there was one documented case of accidental poisoning in the US.[6]
https://en.m.wikipedia.org/wiki/Tetramethylenedisulfotetramine
CsCl is also much less active per gram because about half of fission product cesium is stable (on top of longer halflife)