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
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
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?
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.