BOI TRACON Controller reported an arrival cleared for an ILS approach to a satellite airport turned and descended prior to the IAF and flew below the Minimum Vectoring Altitude.

Narrative

Aircraft X was cleared for ILS Runway 12 with a restriction to cross KICDO at 10;000. I noticed that well before getting to KICDO the aircraft started an unexpected; incompliant turn. I called attention to it to the pilot and offered vectors for the approach. Then seemed confused and asked for the vector. I issued a vector and reassigned 10000. The aircraft was in a 10;000 MVA. Readback was good. I moved to other tasks. I then noticed they had descended below the assigned altitude and below the MVA. At this point the aircraft was entering a lower MVA so I reassigned a new altitude compliant with the new MVA; but the Loss was too late to recover. After establishing them on the approach I issued the brasher statement. I have had numerous aircraft take the early turn prior to KICDO. I have never had one descend until now. Talking to other controllers this is happening to more controllers that just me. It tends to be biz jets not air carriers that make this mistake. Maybe we need to figure out what is causing pilots to fail at properly programming the arc for this approach. Unfortunately; when it comes to pilots being unable to fly procedures properly; the fix for the issue tends to be for us to stop using a perfectly good procedure in order to work around them.