One other factor may result in SAT scores going lower.
I don’t know about other states, but a few years ago Maine’s
Department of Public Instruction changed the high school graduation requirements to include sitting for the SAT exam.
So even if you had no intention of going on to college, you were required to take the exam as a condition to graduate from high school. Oh, and the exam is given on a Saturday.
I suspect the bottom 25% of high school student just blow the exam off and don’t care how they do.