This is part of the passive cooling or power/battery management support implemented in ACPI by the laptop's BIOS. While it's rare (the vendors think they know better), you may have a capability for modifying the configuration within BIOS Setup. You may want to ask Asus for information regarding this feature (i.e. all the setpoints for throttling and blower control) and whether you have any options for changing the configuration yourself.
As for why it is happening now and wasn't earlier, I can think of a few possibilities,
- You've installed a BIOS updates somewhere along the way and this either newly-implemented or modified the (default?) configuration of this feature.
- Your cooling solution has been at least partially clogged with dust and thus the laptop is running hotter. Use a can of compressed air and blow it/them out -- and do this once a month. Note that, depending upon how bad the dust buildup is, it is possible that the compressed air cannot move all or part of the buildup; sometimes (if you've left it for too long) you have to open the laptop up and clean this out by hand.
- Over time, the TIM (Thermal Interface Material, heatsink paste) can dry out and become less efficient in the transfer of heat from the CPU (and GPU) into the heatsink or heat pipe assembly (whatever the laptop has). Replacing the TIM is a complicated tasks but the only way to address this. Now, considering that your laptop has an 8th gen processor, it is not that old and this possibility is thus likely (though its time will come).
Hope this helps,