If the DropDownMenu is open, DropDownMenu_ApplyPosition is called every 200ms to ensure the correct position and has a reference to a DOM element. But if the referenceElement does not exist anymore, the popup jumps to the upper left corner. This can occur in the following situation:
The DropDownMenu-Button is inside an UpdatePanel and the menu is open. If the UpdatePanel gets updated while the DropDownMenu stays open, the reference to the referenceElement is not the same as the element in the DOM. This can be reproduced with a button which makes an async postback in the click event and calls e.stopPropagation() so the menu does not get the click event and it stays open.
Design: Placing the dropdown popup next to the button in the DOM solves the issue, because the popup will be removed when updating the UpdatePanel content.