Water running downhill, with the picture as the landscape. Bright is high ground, dark is low.
Each particle feels which way the slope falls beneath it and takes a step that way. Released in the highlands, they run down into the shadows and pool wherever the ground flattens out.
The trails they leave are the drainage pattern of the terrain. They converge into channels wherever neighbouring slopes point the same way, which is how real river systems form: not by anything digging a channel, but by many paths independently arriving at the same low ground.
Setting inverse runs them uphill instead, which finds the ridgelines.
Because each particle only sees the ground directly under it, it can settle into any local dip and stay there, unaware of the deeper valley next door. The pools are where they got stuck.