A relatively common idea in Sci-Fi and especially speculative evolution is the idea of humans or some other intelligent species ending up losing their sapience(often done for the sake of horror).

While the most common explanation(forced genetic modification) makes sense, I've always been skeptical of the idea of it happening naturally giving how much of an edge it's given us, especially in regards to the idea of it being from some massive cataclysm.

So what are some potential explanations for why a sapient species would end up losing its intelligence?