Probably the clamp that holds the throttle cable in position has loosened up and let the throttle cable move. They are notorious for that to happen.
Some engines are worse than others as B&S and Kohler has their throttle assy on the bottom of the engine, right next to the exhaust & muffler, were all that heat does a job on the throttle cables and the clamp. Kaw has the right idea, the throttle assy on them are on the top of the eng, far from the heat!
All the new mowers I set-up, tightening the throttle cable clamp is part of my PDI after I set the RPM's......along with zip-tying the throttle cables up as far as I can away from the muffler!