Why is laceration reopening after stitches removed?

Many possibilities. Perhaps sutures removed too soon, perhaps you have delayed healing related to any of a number of factors such as renal failure, diabetes, poor nutrition, smoking, perhaps there is too much tension on the wound, perhaps underlying infection. Best to see your doc.
Unknown. A laceration may open up after sutures are removed for any number of reasons. May be due to infection, delayed healing due to medical problems like diabetes or poor circulation. Sometimes the edges of the cut may look like the are coming apart after stitches are removed but the underlying tissue is actually still together. This is especially true in areas where the skin is thickest like the hands.