At first, a wall sit feels still. Then your legs start negotiating.
The position asks your muscles to hold 1 steady shape while fatigue keeps rising.
As some motor units tire, the nervous system recruits and alternates other fibers to keep the knee and hip angles fixed against gravity.
Those tiny corrections become visible as shaking, because the force output is no longer perfectly smooth.
The shake means the hold is getting unstable. It is not, by itself, a scorecard for strength gained or a command to keep going.