I had a colon resection and 1 day home went back to e.r with bad pain resection had come apart. why?

Many possibilities: It's unfortunate to hear that you have had some complications as a result of your recent surgeries. It is, however, difficult to ascertain why you had such complications. There are many reasons for complications, they can range from the complexity of the case, the skill level of the surgeon, or malfunctions of the tools/hardware used just to name a few. There is also just pure bad luck.