Great pointer to the Airliners.net thread, but it raised another question.

It said that they do it mostly with tail-engined aircraft because of the risk of sucking in Foreign Object Damage (FOD) junk with below-main-wing engines.

Is there extra FOD risk during reverse thrust? Or is it just that pushback or powerback happens close in to the terminal/bridge, where there's more likely to be junk? Underwing-engined jets taxi in that area when they come in. What's the difference?