I don't know where that could have been introduced. It's possible that it was taken from a print that had been projected, and somehow the aperture gate or an extra-hot bulb damaged the film, but I wouldn't think that would do the exact same thing to every frame in such a uniform manner. I'd say it's more likely a flaw in the 70mm blow-up process (the workprint was blown up to 70mm by accident and shown in the early nineties - I believe they meant to do a blow-up of the theatrical cut, but something got mixed up and they ended up with the workprint) than anything else - the workprint on the 5-disc set was transferred from one of the last surviving 70mm copies of the workprint that were shown in the early nineties.

It could also be due to the film scanner used to transfer the print to digital, but based on the rest of the set's level of quality I doubt this to be the case.