Commenter Gabrocki had this theory:

I’d blame the temperature difference. Like I said, the nitrogen is heating up from at least -196 °C to around 20-25 °C. That’s being exposed to more than 220°C difference. This sudden heat transfer and the consequential expansion causes a bigger, more sudden explosion. Also, there are 1500 ping-pong balls above and around the nitrogen bottle, which means much of the force coming from the explosion leaves through the bottom, (even though ping-pong balls are relatively light), thus making the bucket jump.

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