There are multiple causes of low sodium levels, according to "Health A to Z":
"...Diuretics, certain psychoactive drugs (i.e., fluoxetine,
sertraline, haloperidol), specific antipsychotics (lithium),
vasopressin, chlorpropamide, the illicit drug "ecstasy," and other
pharmaceuticals can cause decreased sodium levels, or hyponatremia.
Low sodium levels may also be triggered by inadequate dietary intake
of sodium, excessive perspiration, water intoxication, and impairment
of adrenal gland or kidney function..."
The following emedicine articles list some additional causes: