Well, the rate of cosmic “rays” (they’re actually individual particles) hitting Earth’s atmosphere isn’t constant. It varies according to all sorts of stuff, ranging from the activity of the sun to what the stars on the far side of the galaxy are doing. So a sudden and totally unpredictable increase in cosmic rays does happen often enough. And seeing as how these particles are about a trillion times more energetic than anything mankind has made in our particle accelerators, they can certainly do bad things to a circuitboard if one was to hit just the wrong spot; they can change the quantum state of the electrons in atoms making up the circuits, which would of course do weird things to the electronic flow. And since modern integrated circuits are made on a microscopic scale, those small changes can have a major impact on the circuit’s functions (heh, I suppose that the old vacuum tubes would actually be more resistant). That’s always been one of the hurdles in spaceflight. So a cosmic ray particle hitting the computer at just the wrong way is, though not an incredibly likely event to happen, entirely possible.
Think of it as sort of like the EMP from a nuclear bomb, but confined to one particle rather than being spread out all over.
Seeing as how the problem was traced to the flight computer, I greatly doubt that it was turbulance that caused it.
