QuickRouter is intended to find routes for thousands of links fast enough and just selects first path it finds in its unoccupied-space map, which could change shape suddenly while you move nodes around. Set diagram.LinkRouter = new GridRouter() and it should select more stable paths. With it you can also set some costs in RoutingOptions to choose what paths criteria you are after (like staying further away from nodes, bending less, etc).

Regards,
Slavcho
Mindfusion