An employee of the farm in the municipality of Stetten accidentally switched on the methane gas from a pipe in the piggery, local television station Tele M1 reported.
The incident occurred in mid-June but has only now come to light, fuelling concerns from animal rights activists who maintain the pig farm is operating inhumanely.
The Association against Animal Factories Switzerland (VgT) has published photos of the farm that it says shows pigs there have been maltreated.
However, the farmer told Tele M1 that he believes the photos were stolen and that they have been doctored.
He told the station he was upset by the death of the pigs and defended the way his operation is run.
He plans to throw open the doors to the press to show how his farm operates, 20 Minuten newspaper said on Tuesday.
Meanwhile, the farmer finds himself facing threats posted on the Facebook page of VgT, the animal rights group.
The group maintains the operation fattens 1,000 pigs for the meat market and alleges animal protection laws have been violated there.
It said the death of the 135 pigs occurred under “mysterious circumstances” that suggest abundant negligence by the operators.