They will no doubt blame this on AI, somehow (ChatGPT release: late 2022, decline start: mid 2020), instead of the toxicity of the community and the site's goals of being a knowledgebase instead of a QA site despite the design. PS - This comment is closed as a [duplicate] of this comment: https://news.ycombinator.com/item?id=46482620
The downward trend seems to start ~2017, and was interrupted by a spike during the early months of COVID-19. I'd be interested to know what drove that jump, perhaps people were less hesitant to post when they were working from home?
The excess time available (less commute or career pause etc) and more interest (much more new opportunities) were probably leading reasons why they spent more time I would imagine.