What causes a tummy tuck incision to open?

Thank you for the good question.
There are many factors involved when it comes to incision line healing problems after tummy tuck surgery.   Some factors include:  excessive tension upon the incision line closure, excessive blood supply compromise,  suture related problems, infection,  premature or excessive postoperative patient activity, nicotine effects…  Obviously there are differences between plastic surgeons  when it comes to   exactly how the operation is performed ( that may lead to a difference in complication rates).  
 In our practice planning the incision line for tummy tuck surgery is done after careful communication of patients' goals and preferences. I find that marking patients the day before surgery, as much as possible, with the help of the patient's planned swimming suit or undergarment, helps when it comes to placement of incisions…
In most cases, the tummy tuck incision can be kept very low, usually at or below  the pubic hair line (or at or below an existing C-section scar).
The use of the progressive tension relieving suture technique (sutures placed between the abdominal wall flap and the underlying abdominal wall fascia) allows us to keep the resulting scar low and relieve some of the tension upon the tummy tuck closure.  Reducing tension upon the incision line helps keep scars as fine as possible.
Sometimes, especially for patients with a relatively short torso and/or minimal redundant abdominal wall skin, a short vertical scar is necessary ( in addition to the usual transverse tummy tuck scar). This vertical scar results from closure of the previous umbilical opening and prevents excessive tension upon closer in the midline of the tummy tuck incision line. If this vertical incision is not used, patients may end up with high scars, wound healing problems and/or wider scars.
 Patients should keep in mind that the lower the incision line used for tummy tuck surgery, the less skin can be removed at or above the umbilicus.  This may be especially relevant for patients who have stretch marks and/or piercings above the umbilicus.  Sometimes, patients elect to have their incision lines placed higher so that more skin and/or an existing supra umbilical piercing can be removed  during the tummy tuck procedure.  At this point in your recovery, I would suggest that you focus on good nutrition, avoidance of overactivity (I ask my patients to say somewhat flexed at the waist for 4 to 6 weeks), and close follow-up with your plastic surgeon.
 
 Best wishes.