At least in the case of the 200,000 Saiga Antelope, the cause appears to be bacteria/environment related:
> We ... thus concluded that P. multocida serotype B was the primary cause of the disease. [1]
> P. multocida is a ubiquitous organism, most probably widely present in the saiga antelope population in its latent form. The potential pathogenicity is inherent in the organism and can be triggered opportunistically at any time in response to environmental triggers. [1]
I used to think "environment changes" -> "animal populations must adapt, not always successfully". But what seems to be killing entire populations at a time is not mal-adaptation to environment per-say, but an otherwise dormant life-form that we live with every day, suddenly waking up and deciding its time to change the world.
It's not that 3C increase might make the globe too warm to be a comfortable place to life. It's that 3C increase might wake up an incredible evil that has been sleeping for a quarter million years. Evolution is a long-con - some of the billions of strains of dormant, boring, inactive bacterium we've yet to study may actually be the most sophisticated weapons the world has ever developed - just waiting for the right time to deploy themselves. We know there have been mass extinctions we've yet to understand, and we know most of them happened when the world was considerably warmer than it is today.
[1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6537709/