Assuming input voltage is at rated 230V, your output voltage open circuit is going to be at 52VAC (rated output voltage plus the regulation rating of 4%).
Multiply that by root 2, gives 73.5V peak voltage.

Now if your input voltage is at 250, which puts it around 8% above the rated voltage, so your output voltage will be proportionally higher, at 79.4V peak.

Now all you need is a sudden deceleration on a couple axes, and the energy dump back into the DC side can potentially push everything overvoltage.

If you search for my Triac retrofit thread, there's a big post there explaining all this.