Looking at a dogs tail can tell us a great deal about how he is acting just as looking at his whole body can. There are a few different reasons why dogs walk with their tails sideways as I have written another blog post about that.
Among them are:
Checking the anal sacs of a dog
When a female is in heat
A dog that is extremely happy or extremely scared
Dogs that have physically injured their tails.
If you want to read more about dogs' daily tips read here: Dog Daily Tips and Tricks.