In the nucleus of large atoms, the strong nuclear force must overcome or match the electrostatic repulsion between protons in order to remain stable. Thus if the nucleus is too large due to an abundance of neutrons, the strong force will be unable to keep the nucleus stable for long. This requires a change in the number of neutrons, which explains why beta minus decay occurs.
The jump from uranium to neptunium increases the size of the nucleus by one proton, passing a stable limit. The decay requiring the least energy is beta minus decay, which then occurs:
n→p+e−+ν¯e+(1MeV) (beta minus)
p+(1MeV)→n+e++νe (beta plus)