A set of incidents with drum pressurisation was attributed to gas evolution due to bacterial activity.
However on further investigation it was found that the mixture of materials in the drum slowly generated carbon dioxide (max rate 16 ml/h/kg). this could generate 50 psi in a 200 l drum. The drums will deform at 5 psi and rupture at 60 psi.
It is not wise to assume the "easy" answer. Even with the wrong answer, the question should be asked why they kept filling more drums, knowing that they were likely to pressurize?