One common interpretation is that the Qurayzas had broken an agreement with Muhammad and fought on the side of the Meccans, thus making the massacre an act of war. This interpretation is supported by Surah 8:55-58, and while it's debated whether or not the betrayal actually happened, this is the most common explanation.

Another explanation - and one that takes a decidedly more skeptical approach - is that Banu Qurayza happened because Muhammad needed the funds that looting and slavery would provide to fund the war against the larger Meccan army. However, as I said, this is not a majority opinion.

As far as I can tell this event isn't described as a genocide (a highly charged word historians are very cautious about) in mainstream historical sources, only in anti-Muslim media. Please resubmit your question with more neutral wording - I've removed this post.