The main guess researchers have so far is that the damage was caused by cosmic radiation, even though Curiosity was built to survive even harsher conditions.
“The hardware that we fly is radiation-tolerant, but there’s a limit to how hardened it can be. You can still get high-energy particles that can cause the memory to be corrupted. It certainly is a possibility and that’s what we’re looking into.”
The timing is really frustrating, considering how Curiosity has just begun analysing rock samples. Martian samples were “ingested” into the rover’s on-board laboratory last month.