It's the same technique of the "3D touch" the iPhone 6S had, though that was discontinued. Capacitive sensors measure microscopic changes in the glass, as it slightly flexes when you press it. Then a haptic motor does a little kick to simulate the feel of it being pressed down.

They've been doing this since 2015 or so. Prior Macs didn't click on the top 1/4 of the trackpad, because there was basically a "hinge" there and the glass would actually press on a physical button.

Fun fact, the other thing that can cause this is a Bluetooth mouse in a backpack that is being clicked continuously. Confused me to no end for fifteen minutes.

Have noticed exactly this behavior as well, it's one of the better indicators actually the system ain't coming back.