This may be far fetched and the idea overused but what if Damon was convinced that becuase he woke up in his old cell at the end of 4x08, he wondered if everything in the past 60 years was all in his mind? Damon wonders if he ever left the Augustine Society and its freaky expeirements behind. Just like Elena didn't know what was real and what wasn't when she killed one of the hunters, Damon wonders if a witch has casted a spell on him or an older vampire has compelled him to think everything we've seen in the show so far isn't real. Everything is real but Damon doesn't know what to believe and feeling the hopeless of his situation, the idea that he never left (or is back in) the clutches of the Agustine Society is just too much for him to handle and he turns off his humanity. So when Elena and Stefan do rescue Damon they have to get him to turn his humanity back on, this would be a new problem/delimma in the story because they would have little to no idea what Damon went through or how to make him see reason.
Just throwing this idea out there, if it does sound too far fetched or overused, I'd like to know before I publish this as a fanfiction. Please share your thoughts, comments, complaints etc. :)