Dog goes upstairs backwards. Why?

Some people are making fun of this (see video at end of article). They can't understand why this dog is ponderously climbing the stairs outside their home backwards. Some are saying the dog has forgotten how to climb stairs. 

The reason may be far less prosaic and fancy. The likely reason for this apparently absurd canine behavior is that the dog is old and they have arthritis. Their joints hurt when he climbs in the usual way - forwards. 

When he climbs backwards (with difficulty) it is less painful. Logically he decides on the less painful method.

But this is nothing to what I've seen not that long ago: a dog doing walking handstands climbing stairs backwards!! See above. There has to be a logical reason for that too. At the time I put it down to training by the owner. That was a guess. It may be correct. But why bother to train you dog to do it?