I think it's more like slashdot. I didn't leave slashdot for reddit or HN. I just left. It got bad. I'm thinking the same thing is about to happen for me on reddit as well.
At some point, I might be following a whole different set of subreddits than I am today, as old ones die/jump the shark and I discover new ones. But I don't see what would specifically push me to leave the entire site, as long as there are individual active communities there I'm interested in—any more than I would choose to categorically stop visiting blogs hosted on Wordpress.com or categorically choose to never read a post on a phpBB forum again.
Of course, I might leave the whole site anyway, but not because of a push; rather because activity on the whole might drop off in favour of something else, and all those communities I like might migrate away (like many of the chatrooms I visit have migrated from IRC to Slack), leaving a hollow shell of a site that's still, in theory, a host for many communities I enjoy—but not their canonical home, just a secondary one. At that point, there'd be no reason to visit.
It went from me reading a dozen or so subreddits actively to that place I find free amateur porn.
Maybe I just grew up, but I also think a lot of subreddits declined, particularly larger ones.