Is there any way to find out why a dog was disqualified from a particular placing?

They publish them in the Kennel Gazette along with the rule it has broken. Can be 6 or more months in arrears.

And the Kennel Gazette is soon to be discontinued. I assume they will publish all the usual details in the new online edition instead.

It is invariably that it had been incorrectly entered - wrong name or details - wrong date of birth - over qualified or wrong age class.