It's the strangest thing, really, and it boils down to a disconnect between what you see and what you hear. If someone says that a person's name is eluding them, wouldn't you expect them to be watching that name flying around their head as they try to catch it? Or, if they said that they hit the ground running, that they were dropped from a height while their legs were spinning Road Runner-style and sped off the second they hit the ground?
The "it's over your head" idiom always got me when I was younger, and I would actually look up to see if I could find and catch what was over my head. Another classic one was "have a chip on your shoulder" and I would look to see if I could find and remove said Pringle.