One thing is for sure, if you have a sink hole there must be some form of hollow in the ground where the soil is going to. It is quite possible that the soil is going into your septic tank, sewer line or a leachfield line. The only way to be sure of what is happening would be to dig into the sink hole and see where the soil is going. If it is a broken pipe or an opening in the septic tank that can be repaired and the sink hole backfilled.

On my last house years ago I had the same situation. While cutting grass, I stepped and sank into a sink hole. Upon getting a shovel to investigate, I hit sewage water and black dirt, indicating this had been happening for a while.

Turned out my 30 year + clay tile leach field had failed and backfilled itself in with the sandy soil, explaining where my dirt as going.