Researchers moving between companies (and other ways that techniques get leaked) is the largest cause of this IMO. It's happening continuously, so I don't see why the timing makes it implausible. A really underrated strength of Silicon Valley is California's ban on non-competes that allows this to happen and ensures robust competition between model providers both for talent (increasing salaries for workers) and in the marketplace (reducing prices for consumers). If OpenAI had been located in New York instead then Anthropic could never have succeeded, for example.

But I think the other reason you didn't mention is the timing of new compute coming online. Compute is the major factor limiting the training of these models and new datacenter investments are bearing fruit at around the same time.