Dolphin underwater scooter

12V powered propellor SCUBA assistance. Only 2 on/off sw functions , both reed relay+ external magnets work fine. Why so much electonics for isolator function sw and motor on and off sw, no speed control on this one? Assuming it monitors motor overload, what else ? brown out ? Uses 2x ST P60NF0?? TO220 size powerfets and ST maybe LM324N 4x TO92 all unclear under scraped off conformal. A 4mm wide track with central solder thickening overlay, has fused, in a run of uniform (not deliberately necked) track, not at a component. Looks as though the trace has heated to point of melting solder and the underlying copper has split apart by contraction leaving a neat 1mm gap rather than burning up, copper is still bright copper. Is this another failing of PbF ? no reported motor stalling from weed etc , what sort of current would cause this failure?, power devices etc seem ok as far as no shorts and no other hot spots . Normal in-service running current and some PbF metalurgical failure ?