It could be nothing more than the huge amount of rain we've had in the last year, coupled with a high water table.
Septic tanks (as distinct from cess pits) work by digesting the bacteria and allowing the waste water to migrate into the soil through a fan-shaped outlet system.
In a community that I work with, the village hall is on a septic tank and for the first time in living memory (over 50 years) the septic tank system filled to the top during winter. It has never, ever had to be emptied before. The water going through it is predominantly roof soakaway water.