Best guess is you have a wire connection issue. A bad connection causes high resistance, which in turn results in low voltage to the fan, the fan then draws more amps to compensate which stresses the relay.
Troubleshooting involves measuring the voltage at the fan, if it is low then you trace back towards the source in order to find where the missing volts are. If the voltage at the fan is normal, then you would want to put an ammeter in-line to see how much current is flowing, if the current is too high the fan itself could be bad.
