It's not a problem in the yard Henry. It's gravity. Somehow you got yourself some extra gravity around the tank. Or you have exceptionally high localized atmospheric pressure. Or your neighbors are tunneling. Wait, it's gophers! (or the Chinese coming through from their side
)
Back home in Tennessee it was the moles. Our Sassy Girl.........since deceased, God rest her Boxer soul..........was hell on 'em. She'd see the ground move and she transformed into a backhoe.
And at our Tennessee place, shortly after we bought it, I had to replace the entire septic system and relocate the leech field to a different area via 120 feet of 3" schedule 40 PVC from the tank to the field. That was fun.............

But with 30 minute dirt, all I had to do was run 440 feet of 10" graveless pipe for the leech field. Still working great today.