Manchester City are trying to raise the funds to sign Radamel Falcao, which has led them to consider selling Nastasic. Obviously just having cash isn't the issue for City, but complying with FFP is so they need to make a couple sales if they want to sign the Colombian striker.
Nastasic is just 21 years old and a pretty excellent centerback for someone so young. He had injury problems last season, but is already an international quality player and forced his way into City's starting XI two years ago. Spurs would likely have a leg up on Arsenal too because one would think that the Citizens would try to steer Nastasic away from Arsenal, who they would view as a bigger rival.
Spurs don't necessary need a centerback since signing Federico Fazio. He gave the club five established players at the position and even if they sold Vlad Chiriches, they wouldn't need to sign another. But Nastasic is a pretty wonderful talent and if Tottenham can take advantage of City's money issues to get a young player as good as Nastasic, they may be moved to sign him anyway.