I've never heard of just one leach line unless it was some DIY job done 30 years ago.
Anything done in the past 20 years or so would have a d box and several lines running off from that point.
Most likly a plug any place from a vent line from when it plugged up last time to the tank, not from the tank to the leach field.
Why would waste water boil out of the tank then? Waste is entering the tank and not leaving.