I guess I must seem strange so I'll explain. I use the MacBook Pro because I can work on music at home or elsewhere with ease. I love the trackpad and all it's features, pinching and scrolling and gliding, and I find it better for me personally than a mouse, I even get less hand pain than I used to in my mousing days. I do have a Novation Zero SL external controller; but I don't reach for it all the time and I don't take it elsewhere with me too much since it's awkward (like on a train) so I'm in the habit of doing without it except for automation, live, certain tricky things. Most editing and production I can do right on the trackpad and keys (with all the brilliant shortcuts). When I speak of editing and production I mean splicing, lining up, getting sounds. One of the things you do of course is just get that volume fader on an audio or instrument or sub just where you want it, until you decide to move it for some reason. You could have a lot of tracks going and need to build up a sense of your subs and full mix as you go, and so on. In the case of a volume fader, like most things, to move it I grab it with the pointer and drag, unless I'm using the external (it's not as convenient as one might think).
Now picture my hands on the computer keyboard, fingers at the keys. Then I need to use the trackpad to move the cursor somewhere, drag a region, whatever. Being right handed I move that hand down to the pad. The left hand is still at the keys, ready for a shortcut command. But this means that my left thumb is hovering somewhere below the keyboard, which puts it often just over the trackpad. If that thumb moves down close enough to the pad, even just barely touching it, the pad's sensitivity can pick it up. If that happens while the right-hand index is moving on the pad, and a track is selected in the Arrange window, then, voila, you have an unintended pinch or scroll on the trackpad and a consequent effect in the Arrange window.
The effect may be just a visual zoom, but if your fingers are just right you will change the fader volume of the selected track, which of course is destructive, command-z won't put it back if you don't remember where it was. The action is very subtle. If I'm correct, it seems that two fingers on the pad on the same x or y plane will zoom in the arrange; if off axis from one another you get the fader edit. It's impressive that it can be so subtle, but then as a human I can be a somewhat imprecise creature. And besides which - and my reason behind the post - I just do not need the trackpad's subtle action to move a selected fader for me.
You've been very nice to stick with me - I feel like it's just you and me in here. I hope I made myself understood.