Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?
Why When SWR is high, part of the transmitted power is reflected back into the final amplifier, producing high voltage and current peaks and extra heat in the output transistors. Solid-state finals have very little tolerance for that abuse compared to vacuum tubes, so modern rigs include an SWR sensing circuit that folds back drive power once SWR climbs past roughly 1.5:1 to 2:1. The reduced output is a self-protection feature, not a fault in the radio.
Watch out Cutting power does not lower the SWR at all: SWR is set by the mismatch between the feed line and the antenna, so the ratio stays the same no matter how much power you feed into the line.
Foldback protects the finals. High SWR cooks transistors, so the radio backs off to save itself.