Here... I found this while searching on the internet (someone posted this on a forum). Joanie Doss is an Amazon expert, but this happens with all species. Looks like there can be several reasons for it?
Copied from the internet:
joanie doss (her speciality is amazons), in her "can you speak parrot two" book calls it "snow plowing"
here's what she says:
"the birds lowers his head so that the beak touches the groundm& walks forward, much like a snowplow.some birds do this walking backwards, but most will do it walking forward.
there are several reasons why a bird may do this. some birds do it for attention or when they are in a playful mood. some do it to show that they will not hurt you and want you to hold or pet them.
amazons place a high value on food. some do this to locate crumbs or tiny seeds that may have dropped to the floor. this way they can get every crumb!
the most common reason is for assessing footing. this is often done when a bird is put on carpeting & the caroet gives under his feet or when a shadow appears ion front of where he is walking. the bird may think the dark area is an indentation and uses the beak to determine depth. birds with impaired vision sometimes use their beak to help them navigate."