If you have perhaps an electrical fault that can sometimes call for the starter motor to run - loose wire touching something due to vibration while you're driving along - the starter will engage with the flywheel and be driven at very high speed by the engine and act as a generator (lots of volts) with nowhere for the power to go - it will quickly get hot and burn out - you may not hear it happen and probably not smell it - the fault condition may only need to be fleeting, just a blip.
You will not notice the problem until the next time you try to start the engine.
Turn the ignition on and vigourously wriggle the wires in the control circuit, behind the ignition key and around the solenoid - see what happens... Six starter motors in six years is not normal - something is causing these to fail.
Peccavi
October 2008