Is main clutch stuck or something? Normal good practice is every time you leave the seat, put transmission in neutral and engage main clutch. That way you know its in neutral and if anyone bumps the main clutch lever nothing will happen.
If you don't engage the main clutch every time, one day you might not put it in neutral and then a slip and grab for something might connect with the main clutch and away you go! Also having the main clutch engaged stops the pilot bearing turning. Those things usually get little love and may be showing the effects of no maintenance over 50-60 years and just waiting for an excuse to seize.