Hi Andrew,
What application are you using it in? I will need some more information about the experimental setup, e.g., power supply, wiring etc. Depending on the power supply, slew rate, wire inductance, and capacitances at the output of power supply and at the input of converter, turning on the input power supply results in higher VIN spikes at the input of the converter. Were you able to capture the input voltage waveform during power up. That will clearly show if the voltage at the VIN pin exceeded 100V.
Regards,
Vijay