maybe because the first electron has a lower energy. Why does this happen?
The second ionization energy of any element will be greater than its first ionization energy. One way to rationalize this phenomenon is by considering that the first electron is being removed from a neutral atom and the 2nd electron is being removed from a positively charged ion that is already electron deficient. Additionally, Slater's Rules for calculating effective nuclear charge can be used to provide a semiquantitative reasoning for this observation.