Setup:
Runner at third base.

Hitter gets 4 wide and walks to first base.

Normal 1 base walk.

Catcher steps in and throws the ball to first base.

Ball from first base to pitcher all is under control.

Reset setup:
Runner at third base.

Hitter gets 4 wide and walks to first base.

Catcher steps in but doesn't throw to first base.

Hitter sees an opportunity and sprints to second base.

Catcher tries to recover and throws to the short stop.

Ball at the short stop

In this case short stop sees that the runner from third is a bit slow going for home and throws to third base.

Hitter reaches second base (walking double).

Runner from third base doesn't make it back in time and is tagged.

Making it a defensive play.