what makes dogs tuck their tails in and walk off?

Dogs often do this when they are ashamed, scared and simply want to run away from it all. Every time I see pee on the floor and yell, "Who did it?" one of my two dogs will answer me with the tail.

IF he is also shaking then that dog is terrified. If your turing on the vaccum cleaner or loud noises that might scare him then that is why he might be scared. Maby your doing something and your not even aware of it yet.

If they do it a lot, it's a sign that they are scared of whatever is happening. A beaten dog will do that a lot as he's expecting to get hit, never a good sign in a dog. The tail up and wagging is what you want to see most often.

No, it isnt. Usually if the dog puts the tail between its legs, it might be scared or ashamed. If it walks off, chances are it thinks it going to get wrong.
Its perfectly normal :)