Can someone explain to me?

I was shooting my grandpas crossbow for him to see if it was on. It was a good shot but this happened to the bolt can anybody explain to me why?

It wasn't nocked all the way IMO I've seen a few arrow nocks get broken by not nocking all the way

I guess it's possible but I couldn't push it back anymore.

is it a PSE fang by any chance?

the knock could have been to small for the arrow shaft or the string was a bit thick for the knock...could have already had been stressed before hand somehow also

Which crossbow? You shooting half moon, flat or ?? Nocks? Was it the right nock for that bow?

The other knock didn't break but it was a Excalibur crossbow.

This. Excalibur crossbows typically require flat nocks. Was that a flat nock before it broke, or the correct nock for the bow?

For some reason that doesn't look like a flat nock. Be very careful in shooting incorrect nock, as it will void all warranties.

How would they prove what bolt/nock was used?

Is that a lighted nock? Take a pic of the other nocks. Those should be flat.

Looks like a lighted nock, Excalibur should use flat nocks. Nock might have been damaged, not seated on the string good, if not a flat nock it could have been turned wrong... Hard to know from the picture

You would be surprised in what they can see. When I worked at the archery shop, people would come in with an excuse on how they damaged their bow, and usually within 15- minutes, and after talking to them, they would come clean on what they did to the bow to damage it.