My sewing machine is accelerating for an unknown reason
This could be caused by a faulty capacitor in the machine.

See glossary item on this website:


  Electrical sewing machines have a component inside the motor of the machine called a capacitor.  This helps eliminate electrical noise from the motor and prevents the electrical noise from going back into your homes electrical grid.  When capacitors age they can split open and leak electrolytical fluid, which renders them faulty.  Once this happens, the motor of the machine will at times start running by itself.  The best course of action for when this happens is to take it in to get it looked at by a qualified sewing machine technician.


from: https://sewingmachinecentre.nz/pages/glossary#Running-On