The twitter update TST posted says it all:
According to a Raiders source Mark Davis pulled plug on Saffold deal. Could have had surgery and been ready for training camp. Mark said no
Here's some "semi-inside" information for you. During an NFL physical exam, they normally do X-rays on injuries of concern. Usually, X-ray is good enough, but if it's a soft-tissue injury (like most sports injuries), then an MRI is warranted.
One of the most challenging aspects of MRI is that it's so ambiguous. Little spots show up all the time that could be cancer, or could just be an artifact from a stray magnetic field. You can find almost anything you want on an MRI if you look hard enough.
My theory is this: Mark Davis realized that his GM was an idiot, replacing incumbent LT Veldheer with a somewhat inferior player (in terms of durability and overall talent) at a higher price than Veldheer asked for, and put the kibosh on the whole thing. He could have easily told the team doctor to find anything he could, any kind of abnormality that would justify a failed physical.
I'm not saying that's what happened. Just pointing out that there's a very plausible way for things to have happened.