FWIW my take is simply slow sealing of the case mouth in the chamber at firing.
As other have mentioned, this is usually evident at the shoulder or upper case body, and it happens every so often with my .400 Jeffery using "standard" regulating loads of AR2213.
Nothing to worry about IMHO; if in the case body or shoulder they pop out when next fired, if in the neck they pop out when sized.
Having said that, if they happen frequently, I would neck-ream the cases if thicker than standard, or at least neck-anneal, as they are possibly too hard in the neck area to seal properly. This is good house-keeping practice for case longevity anyway.

Of course, it could be the opposite....

Like Daryl implies, a partial ignition or 'low pressure event' will usually show up on the target. A minor chamber-sealing shortfall at the case-neck will likely not.