As you can see in the photo for step 10, the trackpad is held with screws that attach along the side of the trackpad that is adjacent to the spacebar. The other long side is held by the two tabs along the bottom. Once you open up the computer you may find that one of the tabs on the trackpad is bent/broken, or that some supporting part of the lower case is cracked. As far as I know, there’s no easy way to tell until you get in there — sorry.
One other thing that you may find once you get inside is that your battery has started to swell. The battery is directly beneath the trackpad, and when it swells it pushes up on the underside of the trackpad. Since the battery may not swell evenly, this may create a high spot that the trackpad teeters on when you’re pressing on it. Another symptom of a swelling battery will be if the trackpad works intermittently or only in some areas, or will track but won’t click. This will require replacing the battery - which if it has started to swell, you want to do anyway.