What triggers this behavior puzzles me too: I haven't paid enough attention to what I'd done before to notice the triggering pattern. About the only gesture I use is the 2-finger drag for panning. I use the UI Area/Zoom/Pan/Reset commands frequently and I tend to do most of my work in the 2D viewports. I don't think I've ever used the pinch-zoom gesture you mention. I know, makes me sound like a luddite! ;-} I'll try to pay more attention and let you know if I notice a pattern.

Here are my MacOS Trackpad settings:

Point & Click
Look up & data detectors: Tap with three fingers
Secondary click: Click in bottom right corner
Tap to click: Tap with one finger
Force click and haptic feedback (checked)

Scroll & Zoom
Scroll direction: Natural
Zoom in or out: Pinch with two fingers
Smart zoom: Double-tap with two fingers
Rotate: Rotate with two fingers

More Gestures
Swipe between pages: Scroll left or right with two fingers
Swipe between full-screen apps: Swipe left or right with three fingers
Notification center: Swipe left from the right edge with two fingers
Mission control: Swipe up with three fingers
App expose (unchecked)
Launchpad (unchecked)
Show desktop: Spread with thumb and three fingers

I think most of these are the defaults & I seldom make changes. I don't use most of the gestures. Probably the most important to me are the Secondary click and Tap to click. While writing this it occurred to me that I've been occasionally surprised by the Notification center showing up & wondered if that might be a precursor to this behavior. I don't normally intend to use that gesture (Swipe left from the right edge with two fingers) to see the Notification center. A quick test and it didn't seem to trigger it.

--Larry