Its wrapper offset.
Everyone has made valid comments :
The wrapper itself can cause offset but its very,very rare 99.99% of the time its about having a level of light that exposes the film through the wrapper and that usually only happens on red window camera's. When red window camera's were made film tended to be 'less sensitive'. Temperature and humidity, static and storage as other people have said can also contribute.
So, its really about 'good practice' : If you have a red window camera and it has a cover, and many have, USE IT, if it does not, make one using black electricians tape, ensure the camera is 'in the open' for as little time as possible. When you have finished using it ensure it goes back in its cover and then back in a light proof camera bag. Try not to have exposed or unexposed film in a red window camera for too long, when loading 120 film ( in any camera ) load in subdued light, never in bright sunlight.
As CmacD mentioned, our wrapper is printed with a fairly light print this helps avoid offset, some people say its difficult to see....we consider that worthwhile to help avoid wrapper offset which is pretty catastrophic and unrecoverable..
Simon ILFORD Photo / HARMAN technology Limited :