Both Pierre Igot and John Gruber cover this topic in great detail; the fact that the first user interaction with Safari’s tabs while dragging ultimately determine their ability to either reorder the tab and/or open a new window/tab. But that’s generalizing it far too much.

If you’re an anal Safari nut, like me, I suggest you read both of their articles.

Source: Safari 3.0: Dragging tabs up or down to move them sideways.
Secondary, source: Safari’s Tab Dragging Modes.