Since some dogs' ears are not naturally pointed, it seems sense that when they feel at ease, they would be lay back in that manner. Your dog may be pleased if their ears are back but not pressed flat against their skull.
One of the most common interpretations of a dog flinching back its ears is that it is afraid or at least leery of something.
This may be particularly true if you observe this cue along with other signs of "fear" in the body language.
A dog holding its ears back, particularly if they are pressed down flat, may be preparing to bite. This will frequently be accompanied by terrified or hostile body language.
When in agony, dogs may pull their ears back out of dread of being wounded much more if they are approached.
In very uncommon situations, a dog with a particularly bad ear infection may be pulling their ears back out of pain. They could be protecting them from further harm by holding them back.
A dog might pull its ears back occasionally just to hear what's going on behind it better. If you and your dog are in the garden and another family member calls them from inside the house, you might notice this.