What Causes the Center Crease in Abs After Tummy Tuck?

Are you talking about the vertical crease or the horizontal crease.
The vertical crease is typically due to a natural thinner layer of fat beneath the skin. Insetting the umbilicus in a tightly repaired abdomen will oftentimes accentuate this vertical crease. Occasionally during surgery, patients have an excessively thick layer and a thin strip of fat is removed to create this crease. This is similar to abdominal etching.
The horizontal creases are due to natural creases in the rectus abdominis muscles called tendinous inscriptions. These are generally only visible in thin patients and can be more pronouced from repeated flexion (situps). These can be artifically created using abdominal etching.

I personally try to create a vertical groove in the upper abdominal midline by dissecting the tissues with my finger which creates a separation of the fat and brings the skin closer to the muscles making a more muscular look. I also use a specific dressing for the first week to accentuate the groove. You can see this effect in my tummy tuck photos at: