Yes could be imbalance but is also an in-between gait - e.g. when leashed and going faster than a walk but not as fast as the dog’s natural trot a dog may pace. If you try running with a fast dog and can’t run as fast as they would naturally, you might find the dog chooses to pace to stay with your speed.
There are a couple of breeds that have atypical gaits - the Old English Sheepdog is known for its “bear-like” gait, and the breed standard says “May amble or pace at slower speeds.”
The Bulldog also has a distinctive and unusual gait - the breed standard says "The style and carriage are peculiar, his gait being a loose-jointed, shuffling, sidewise motion, giving the characteristic “roll.”
My breed, the Brittany, is known for over-reaching at the trot - the correct dog will trot so that the “hind foot should step into or beyond the print left by the front foot.” For a lot of other breeds - this would be a fault. The Brittany is supposed to be square (legs as long as the body) so a dog at a full trot has to overreach.
There are probably a few others that are known for unusual gaits. Most are because of an unusual proportion in some way. Those are the only ones I know off the top of my head…but most breeds will pace sometimes.
A dog that never trots - that’s usually an imbalance somewhere.