What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?

When strong local AM broadcast signals combine to create spurious signals on other bands, it is often due to an external non-linear junction acting as a mixer. This phenomenon is commonly known as the 'rusty bolt effect.' Corroded metal connections—such as rusty fences, gutters, or guy wires—form unintended diode-like junctions. When strong RF fields from nearby broadcast stations hit these oxidized junctions, the signals mix together, producing sum and difference frequencies (intermodulation products) that are then reradiated into the MF or HF bands. Distractors like receiver overload or over-modulation don't describe the physical mixing of multiple broadcast signals occurring out in the environment before they even reach your antenna.