The beta electron comes from the involved W-boson which is one of the force carriers of the weak interaction. This boson decays and yields the electron along with some other stuff, but let us keep it simple.
The fact that this W-boson exists really has to do with the energy involved in these decay-reactions. It is a bit like two particles sitting on a mattress. the mattress will vibrate and this vibration corresponds to a particle of certain mass and energy. This particle really is the exhange boson that represents the interaction between the two particles that sit on the mattress. This generated particle will carry momentum and therefore it really represents and force.