Diagnosis
The fan isn't "random"—it's being triggered by the auto fan feature. This model has a sensor that turns the vent on when it detects heat or steam from the cooktop below, and it can be oversensitive or stuck on.
How to Fix It
You need to turn off the auto function. Press the "Vent Fan" button on the control panel until the display shows a setting you want, like "Off" or a manual speed (Low/High). Many people don't realize the button cycles through Auto, Low, High, and Off. Hold it down for a few seconds if tapping doesn't cycle. The auto light should go off.
If That Didn't Work
If the fan still kicks on after being set to Off, the control board might be sending a faulty signal. A power cycle can sometimes reset it: unplug the microwave for two full minutes, then plug it back in and immediately set the fan to Off.
Heads Up
The auto feature is great for catching stove smoke, but it uses a heat sensor that can get coated in grease. If it's overly active, clean the grease filter and the underside vent intake thoroughly. A dirty sensor reads ambient heat wrong.