There is an initial position for non-modal popups which is always the same.
If you open multiple non-modal popups they will each be positioned the same distance down and to the right from the previous popup creating a cascade.
The reason for the popup cascade is that otherwise there would be no visual indication of multiple popups being open without the user actively moving or closing the top-level popup.
Once you have moved (or closed) a popup the system no longer uses that popup for reference and the position it started in is now an 'empty slot' where a new popup can be positioned.
The last popup to be opened will always be placed on top.