Supposedly Tim Sweeney saw the writing on the wall in 2012 as Epic sold off the Gears franchise to Microsoft that "free-to-pay" "Games as a Service" were the future of the industry and decided to shop around for expertise in that area. Tencent already had expertise in that area (both homegrown in China with several successful "GaaS" games and in already owning Riot Games at that time), and the investment was also supposedly a signifier of a bidirectional exchange of expertise sharing.

One primary source linked from Wikipedia: https://www.polygon.com/a/epic-4-0/the-four-lives-of-epic-ga...