What did dred Scott owner do?

The owner was an Army Doctor Who had spent most of his career near home in slave-country. When he was sent to a Northern posting, he took his slave with him. At that point, Scott could have applied for his freedom, and it would have been granted automatically.

For some reason, he didn't, and the two of them eventually returned South.

When the doctor died, Scott was left to his brother-in-law in his will as property. Scott then tried to apply for his freedom retrospectively. The local judges had never dealt with this kind of application before, and that is why it reached the Supreme Court, where it became a highly controversial case.