Why does positron decay occur?
A positron is the antiparticle of the electron, i.e. it has a +1 charge and the same mass as an electron.
Here's an image illustrating the difference between positron decay and beta-minus decay
Notice that nuclear transmutations take place in both cases, and that each type of beta decay brings the initial ratios of protons to neutrons in the nucleus to 1:1, this being the main reason for their occurrence.
Here's a link to a more detailed answer other contributors have posted: