Payroll robbery?
Probably a moneylender's hoard, something like that. Roman banking was all private party lending.
There were also many intermittent rebellions and mutinies from episodes of Roman cruelty and misrule. The Roman imperial system was prone to such disorders.
I have heard that before going off to battle the pay master of a Roman Army Unit would bury the money in a secure location until they returned and then and only then would they get paid (no sense in paying a dead guy). If the pay master died the location of the money died with him.