Titanfall 2's playerbase has seen an increase of over 650% in two days, according to SteamDB. There are multiple factors that contributed to this increase.
The one that points most toward it was a generous sale, in which the game was 75% off on Steam. Meaning that anyone that had the game on their wish list would have received a notification indicating that the game was now cheaper.
Another factor is the intersectionality between the Apex Legends franchise and the Titanfall one. Respawn has said that Season 9 of Apex Legends will include more material from Titanfall that will come to the game "one way or another."
This thought has been followed up by the game director, Chad Grenier, who went on the record to say "I've told some people before, if you're a Titanfall fan, hang on for Season 9 because there's gonna be some really cool stuff there."
On top of that, the Valkyrie reveal trailer launched on April 19 and there were multiple references from Titanfall scattered in the trailer, which has been shared and seen thousands of times by the community. As of now, it Is not clear in what way Titanfall will appear in Apex Legends.