I too can't imagine why on earth anyone would crop a Basenji's tail! Perhaps the tail was damaged?
I do know that the Liberian black and whites which have now been crossed with Central African Basenjis, occasionally had their tails cut by native Liberians to avoid monkeys grabbing them by their longer tails. This was some years ago and I don't know whether they are still kept for hunting in that country.
Perhaps someone who knows modern day Liberia could say?
That would make the only sense. I've seen videos of monkeys swinging down from trees and grabbing a tigers tail repeatedly. Also grabbing their ears. The monkeys were lucky as the tigers came really close to grabbing them for lunch. 😃