horse bucking while galloping

Question
QUESTION: hello sir, may i know why few horses buck while galloping??

ANSWER: A horse will buck or kick out when starting to cantor if the basic foundation of the horse is not there. If not properly trained from the beginning, the horse will resist cantering with a rider when as to do so.

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QUESTION: sir, what do you mean by "basic foundation" and "horse will resist cantering with a rider"

Answer
The basic foundation of the horse is what all the following training is built on. It is like learning about all the letters in the alphabet before you can write a sentence or a paragraph.
Resisting cantering is when a horse does not want to canter. They will stay in a trot or kick out and sometimes try to buck.