Here’s a good general article on regurgitation in dogs.
https://wagwalking.com/condition/regurgitation
If your dog is generally acting normal (active and alert with no signs of pain) and isn’t losing weight, I would guess that the cause is benign (eating too fast, a mild hernia, etc) vs dangerous (cancer, megaesophagus, etc), but it’s still a good idea to investigate it fully, just in case.