During Mitosis in the cell, the nucleus membrane must dissolve to allow the spindle fibers that pull apart the sister chromatids to the daughter cells access the chromatids. While this is required for mitosis, it poses a risk to the genetic material of the cells.
The nucleus membrane acts as a barrier from the cytoplasm of the cell to protect the DNA and RNA from the material that may cause damage to the genetic code. During this period, therefore, the "risk" for mutation of the genetic materials are high.