A bit of an over-simplification. A fair trade could cause the stock to rise if people think it will likely be unfair, or if people think that TikTok will be run better and more valuable in MS' hands, or that there's some synergy with other MS products/users to take advantage of, or because lots of investors might not move money from American stocks to Chinese but are happy to move money from being invested in other US tech companies into MSFT, or... etc.