This is a tricky one to troubleshoot. Looking at the nature of the problem, there's a few possible causes that come to mind:
* Problems with the backing paper interacting with the emulsion. OP says the film was stored under favorable conditions, so it's a little less likely to be the cause, unless it's a manufacturing/materials problem.
* It is possible that condensation occurred on the surface of the film when loading the film into the camera. This is most likely to occur if the film was stored refrigerated and not allowed to come to ambient temperature before loading it into the camera.
* Particulate contamination of processing chemistry could result in this kind of defect pattern.
* Manufacturing/Q&A problems in the film/emulsion itself. Given the brand and type of film, this is somewhat far-fetched.
It does not look like air bubbles in the processing chemistry, chemistry volume or agitation problems to me.