I don't know the story behind it, but fortnight was adapted into a PUBG type free-for-all mode which seems to have helped its popularity. Being able to jump on a trend like that so quickly shows impressive flexibility. They didn't need to make a whole new game to cash in, the ROI is probably crazy.
Yeah I would say Fortnite was influenced by Overwatch and PUBG and maybe even Splattoon, but originally probably started out as Epic's Overwatch type game that Blizzard was successful on.
