This is something that happens rather than something exceptional. Sometimes an incision breaks open. This may happen along the entire cut or just part of it, and your surgeon may decide not to close it again with sutures.
That last part is worth sitting with, because it answers a question people often do not ask out loud. An incision left open after it has broken down is a decision, not an omission.
As for why, several factors can contribute. Obesity increases the risk of infection after surgery, and being overweight can put tension on stitches, which can make them break open. Smoking increases the risk for complications such as infection and wounds breaking open. Infection itself can make a wound larger and take longer to heal. Other factors include diabetes, poor blood flow, older age, heavy alcohol use, stress, and certain medicines: corticosteroids, NSAIDs and some chemotherapy medicines.
There is also one mechanical cause. Soaking a closed surgical wound could cause it to reopen or become infected, which is why showers are better than baths.
None of this identifies why your wound opened. A list of contributing factors is not a diagnosis, and more than one of them is usually present at once. What is clear is what to do about it, and that is on this page under when evaluation matters.
What happens next is simple. If the wound is not closed again with sutures, it will heal from the inside to the outside. A dressing helps absorb drainage and keeps the skin from closing before the wound underneath fills in.