There are people with giant hiatal hernias and no GERD and there are people with bad GERD and no hiatal hernia. Hiatal hernia seems to at least make GERD worse, but is not the only factor and the causal link between hiatal hernia and GERD seems to run both ways (i.e. people with GERD also develop hiatal hernia over time instead of just hiatal hernia causing GERD).

Other possible explanations found in the literature are at least: small intestine bacterial overgrowth; dysbiosis of the esophagus; food intolerances; postural problems; incorrectly (for some reason) functioning LES (or other related) muscle etc.

It's possible that all of these contribute a little bit to GERD and whether you get problems depends in all of these factors in some non-trivial way.