What Causes Upper Tummy Bulge After Tummy Tuck?

If a mini tuck was performed with no tightening of the fascia, this could cause a bulge in the upper abdomen. Mini tucks can be performed with full fascial tightening (I e hybrid tummy tuck (TM). Still even if a beautifully performed full tummy tuck was performed bulges can result. Remember that the patient's fascia is aggressively tightened, not completely replaced with a young fascia that has never had the stretching of childbirth. Your doctor should be realistic with you. Not every abdomen will be flat after a tummy tuck if there is fat inside the abdomen or the fascia is especially loose.

I like Dr. Beraka's answer, so I will not repeat his excellent summary.
I would just add that when you take fat out of the body, via lipo or Tummy Tuck, the body readjusts its total fat content according to the "calories in, calories out" balance.
If there is no change in diet or activity level, the body will continue to store fat, but since the fat cells in the lower abdomen are gone, it will look to store fat in nearby areas. The upper abdomen is a common place for the fat to go.

This can be caused by loosening of the stitches or weight gain. Despite our efforts to tighten the abdomen as much as we can, certain events can happen that reverse this tightness. An abrupt increase in abdominal pressure with severe coughing can "pop" some of the stitches. Gaining weight will cause the loose areas of the abdomen to bulge as well. I hope this helps!