His immediate reaction was an irritated "What are you talking about? That's..." Then his expression sobered, even became grim.
"I did something wrong," he surmised. "They took you away from me." He must have made a monumental error in judgement; the Council only took a Padawan away from his Master if they were certain the Master's tutelage was going to have an adverse affect on the Padawan.
"What did I do?"
Because it had to be that. Obi-Wan was, deep down and (mostly) secretly, always concerned that he wasn't instructing Anakin adequately. That he'd fail the bright, restless, brilliant boy somehow.