What causes the fusable link between the starter and ignition to burn out?

Real common when positive and negative make contact.The fusible link is a safety mechanism to prevent further damage to a circuit.It sounds like the ignition circuit, solenoid to starter, switch to relay , relay to solenoid has a short or a hot wire is grounded somewhere in the starting circuit. Sometimes the spring in the solenoid sticks, causing the starter to heat up and melt the link.Also if brushes in the starter motor are worn the hot and ground contacts in the motor come into contact, that will melt the link. Or the winding in the starter or relay ground out causing the fusible link to melt.