I think this usually happens because some SEO methods focus on quick gains instead of long-term value.
In the beginning, search engines may not fully detect patterns like unnatural links or over-optimization, so rankings can increase quickly. But over time, as more data is collected or updates roll out, those signals get picked up.
Another reason could be lack of consistency. A site might grow fast with a few strong pages, but without ongoing content updates or authority building, it starts losing momentum.
From what I’ve seen, strategies that focus on real user value and steady growth tend to be more stable in the long run.