Some Clues (Thanks to JimO)
From the first time this happened back in 2003
From the May 6, 2003 New Scientist article:
"It's looking like a software error," Oberg told New Scientist. "That's based on the crews' account of how the computer mode changed without any input from them."
Back-up system
The computer system aboard each Soyuz is designed to automatically orient the craft so that it enters Earth's atmosphere at precisely the right angle. If this system should malfunction, it will switch to a less precise back-up orientation mechanism.
Yury Semenov, head of the Energia Space Corporation, which builds the Soyuz TMA, said a member of craft's three-person crew could have accidentally deactivated the orientation system. But Oberg's NASA sources did not suspect human error: "They have reviewed the accounts of what the crew did and see no incorrect actions," he says.