The way I think about it, is air cooling and lack of access to that air.
It does not need to be real hot weather to cook up a saw- I have done so in below 20 degrees Celsius!
Ride an air cooled dirtbike on a hot day and the air is being forced over/through the cooling fins of the cylinder as you ride- cake that cylinder in mud and let it dry on- see if the results are the same.
A dirty saw with cylinder fins packed full of dirty oily sawdust wont cool as efficiently as a cleaner cylinder will.
Work a big saw hard with a big bar buried and muffler up against the log face- won't cool very well.
Couple in stinking hot air in the first place and heat transfer from fins to air is not great.
Saw tuned for cooler conditions, then worked hard in hot conditions- not great.
Substandard oil or not enough oil in the fuel mix, stinking hot day- not good.
If your saw is up to the conditions, tuned to the conditions and sawyer is not a complete numbskull- you can saw in most any weather.
Overtax a saw and create the wrong conditions and you can burn one up in most any weather as well.