it's real scarry that you have to ask.. but ..
your starter is staying energized.
now. this toggle switch. is it powering the ignition coil and some aftermarket solenoid that starts the tractor? and is wired wrong?
are you still using the oem 3 terminal solenoid and push button on the trans for starting? if so.. make sur ethat lil wire is not touching ground... it will cause the starter to energize via the solenoid.
lastly. sometimes the solenoid hangs or welds the contacts.
get on her.. put in neutral. hook bat cable up. if starter spins, bang on solenoid, after checking wireing first. if poping the solenoid fixes it. you know it was hung / welded. if it does it again.. repalce it.