Thanks, missed his link.
Wow. Well, I can't imagine it was a health issue, because they could have diverted any number of places with great medical care other than SFO, same for a homeland secuirty issue: they would have landed in as isolated an area as possible and not flown a 747 loaded with fuel back to a major metro area.
I guess they wanted SFO mechanics to look at something that wasn't immediately threatening.