Hi Audiofun, all:
The fundamental cause is that cartridge suspensions (particularly the elastomer dampers) change in mechanical characteristics as a function of operating temperature, and these characteristics also change with humidity. The temperature dependency is why you will see references to "temperature-stable" suspension and damping systems in cartridge literature from Denon, Highphonic and others.
The 11th figure on the following page compares the temperature dependency of Denon's DL-1000A with other cartridges (presumably also Denon-made).
https://www.denon.jp/jp/museum/products/dl1000a.html
Although "temperature-stable" suspensions are less temperature dependent than other suspensions, they are not completely temperature independent. Some effect of temperature on cartridge performance still remains (and you can see this in the same Denon figure).
In my own experience, silicon rubber does combine acceptable damping performance with temperature stability, but it tends to not sound very good, so you probably won't find many cartridge designers using it.
The manufacturer of your cartridge may specify a preferred operating temperature and relative humidity. Chances are that you will be able to reduce the warm-up time and get better sound overall if you heed those recommendations.
FWIW, the frequency response of phono cartridges also changes depending on the LP groove radius - the deeper the cartridge goes in towards the label area, the more the top frequencies will be attenuated (by enough to be clearly measurable).
kind regards, jonathan
