Shield Sovereigns
The term "shield sovereign" is usually applied to a Victoria young head sovereign, minted between 1838 and 1887, with a shield as the design on its reverse (back). There were also shield designs used for George IV and William IV, and also in 2002 for Queen Elizabeth II's Golden Jubilee year. There is no particular need to refer to any of these other reigns as "shields", because no non-shield sovereigns exist for the same dates ranges, whereas for Queen Victoria, the St. George and dragon design was re-introduced in 1871, paired with a young head obverse until 1887.
We can understand inexperienced collectors adding the word "back", but when dealers add it, we cringe!

Tautology
Tautology means saying the same thing twice.
Because there are no sovereigns with a shield on the obverse (front), then it is unnecessary to state that it is on the back. We can't really understand why so many people say "shield-back sovereigns", unless it's because using more words gives them the chance to hear their own voice for longer, or it makes them feel more important to use more words than necessary.
One of our other favourite, or should that be unfavourite, examples of unnecessary verbiage is the expression "at this point in time" or sometimes "at this moment of time". The word "now" would usually substitute perfectly well, and save four words. It strikes us there are two possible reasons for using the longer way round, one is when politicians or other spokesmen wish to play for time while they make up a suitably plausible answer, and the other is because they presumably makes them sound more important because they have used more words. We think it simply makes them look more stupid, or possibly dishonest.