He actually kinda checked out with them after I think he got frustrated with their lack of progress/ambition. One of their fans' biggest criticisms of him was that he tried too much with his dribbling/passing when he should keep it simple. Reading between the lines I think he tried his best to be their talisman and driving force but he found it difficult being in a losing team rather than a winning one (and he's not exactly Kevin De Bruyne!).
Not sure what discussions went on behind the scenes but as soon as Soton found out he was going to run down his contract to play for a bigger side they got him out of the team and started life without him. His form was dipping before that anyway, which I assume was motivational.
If you look back to his first season with Saints he was extremely influential and effective, much like what we are seeing here at Spurs. Except his positional discipline here seems even better as he doesn't see himself as a big fish in a small pond and leaves our other players to do their jobs.
Just my take on his recent career. Must say I really did like the look of picking him up in the summer and has turned out a great signing so far.