I cannot guarantee you that but there are articles that say it was due to the earnings miss
https://www.marketwatch.com/story/activision-stock-falls-8-after-earnings-miss-2018-11-08

And other sources say it was due to annoucement of diabblo
https://jogos.uol.com.br/ultimas-noticias/2018/11/06/acoes-da-activision-blizzard-caem-apos-anuncio-de-diablo-immortal.htm

The earnings miss is enough to get a huge drop on the stock price, but the announcement might have an effect on that. Also, Notice that both news were realesed before the market open on the 9th of November.

Thanks for the upvote!

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