Prevent MacBook Pro from turning on by touching trackpad
Same situation here - shut down the MacBook 2018, then wait a few minutes, touch trackpad or any of the buttons and it starts booting up. Very annoying.

Found the answer here Turn on (start up) your Mac Looks like it's the expected behavior. From their support page:


  Additionally, MacBook Pro (2018) and MacBook Air (Retina, 13-inch, 2018) turn on when you press any key on the keyboard or press the trackpad.


Which is exactly my case.