I think there are a combination of factors intermingling, situations like the API backlash just jostle things a little harder and that's when you see big spikes. Once a platform like Lemmy begins to see more and more traffic and, in turn, content, it starts to become a viable alternative.

Lemmy existed for at least a couple years before I joined, for instance, and I came with what I would guess was the biggest wave so far (June 2023). Provided the userbase can keep up a respectable momentum generating discussion and content, the next wave could be bigger or it could be more resistant to leaving because there's enough content here to consume and interact with.

Reddit could take years to lose substantial portions of its userbase or it may shed some and stay solid, but Im not one of these people who obsesses over it's ruin. If they survive long term, God bless, whatever, who cares. What's interesting to me is seeing an alternative sprout up and actually generate traffic and start building a community, whether that's Lemmy or something else built on ActivityPub or something else built on a different federated framework or even something else entirely that's centralized... I think Lemmy is one permutation of this and it has undoubtedly got some traction.

I sometimes wonder if/when I'll start getting random Lemmy links from people instead of ones to Reddit.

edit: I should also add that considering reddit is trying hard to get value on paper and probably still hoping to ipo, we probably shouldn't put it past them be shitty once again at some point in the future.