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,
Hope this helps,