Okay, I figured it out (I think). If I click and drag a window and bump into the line at the top with all the drop-downs, it resizes to full screen. Dragging slightly down from the top resizes to previous. If I drag to the right or left and my cursor hits the edge of the screen, it resizes smaller, and reverts to its former size when I drag it back toward center.
The left-right thing isn’t terrible now that I know what’s happening, but I don’t like the resizing full screen.
The only thing I’ve installed is BetterTouchTool to disable the touch bar. You can modify the trackpad with BTT, but I haven’t done so.
I’m surprised no one else has experienced this–it doesn’t seem like a bug, nor do I think it’s because of BTT. It’s another “feature,” it seems.