Its because the layout system has also a physics system.

>A force-directed graph is a technique for visualizing networks where nodes are treated like physical objects with forces acting between them to create a stable arrangement. Attractive forces (like springs) pull connected nodes together, while repulsive forces (like electric charges) push all nodes apart, resulting in a layout where connected nodes are closer and unconnected nodes are more separated

https://observablehq.com/@d3/force-directed-graph/2