Why do my friends open the door and step back?
TMT: When did you last shower? If the answer is recently, our best guess is that they are trying to give you a not-so-subtle hint to enter their apartment before greeting them. In Russia, if you try to kiss or shake hands with someone over a threshold, you may as well be giving them a slap in the face.
According to Russian folklore, a house spirit lives on the threshold of each home and any interference of a hand-shaking nature will result in their wrath/bad luck/a potential plague. In the future either gently away — allowing your host to exit their home — or follow them a couple of paces into the room and away from the danger zone. Then shake hands, kiss, hug or perform whatever greeting you desire. The house spirits won't mind.