Hi Andrew,
Since you're setting the margin to be 1000px, the RadComboBox is appearing towards the bottom of the page. To quote the RadComboBox documentation, "The drop-down list automatically expands to the opposite direction when the screen boundaries would otherwise be crossed". This is why the box is opening to the top as opposed to 'dropping down'. In regards to solving your issue, I would recommend setting the margin to a smaller number so the RadComboBox will 'drop down' as opposed to 'jumping up'. To keep the list from opening past the top or bottom of the page, all you need to do is set the MaxHeight property to a reasonable size, maybe 150 - 200px:
Hopefully I could be of some help to you! Good luck,
Master Chief