Why did the Japanese yell 'bonsai' during World War II?

The short answer: they didn't. They did use the Word "banzai" (which may sound somewhat similar to those who don't speak Japanese. This means "ten thousand years" and is part of a longer phrase along the lines of "ten thousand years to the Emperor."

The Word bonsai is very often mispronounced in English. It should sound something like the French Word "bonne" and the English Word "sigh" together.

I hope this answers your question. :-)