When the touchpad is set to emulate a mouse scroll wheel while using a circular gesture, swiping clockwise is scroll down and swiping counter clockwise is scroll up. Of course, you can change this if you want:
Steam>Settings>Controller>Pair and Manage>Desktop Layout>Edit>Edit Layout>Trackpads>Left Trackpad Behaviour>[gear icon]>Invert Swipe Direction
Or just swap the "Clockwise Command" and the "Counter Clockwise Command" under Steam>Settings>Controller>Pair and Manage>Desktop Layout>Edit>Edit Layout>Trackpads>Left Trackpad Behaviour
But regardless of the fact that it can be changed, there must be some logic to why the Steam Deck touchpad behaves that way by default, right? Help me make sense of why some dev team at Valve decided that the default should be "clockwise=down" despite these other common scenarios in which "clockwise=up".
The scroll bar is on the right so this is intuitive to me.
Don't think of it mimicking a scroll wheel, think of it mimicking a knob. The right side of the knob goes down when turning clockwise. If the scroll bar was on the left it would correspond to the left side of the knob and clockwise would be up (or at least should be).
I know not everyone would find this intuitive. My wife and I both bought the same make of car twice in a row (before wireless car play and android auto so we used the native ui) and the new ones moved the scroll bar on the infotainment from the left to the right, making clockwise change from up to down. I adapted in a day because it was obvious that moving a slightly curved bar on the right is clockwise for down but it frustrated my wife for several months until she got used to it.
Edit: also volume generally goes left to right which tracks with the top of the knob for me so clockwise is up.