nuclear physics - How can beta plus decay be possible?

$\beta +$ decay is where a proton gets turned into a neutron and a positron and a neutrino.

However, a neutron is heavier than a proton, so obviously this reaction is endothermic. So then, why does it happen? I've seen an explanation here in the question How can a proton be converted to a neutron via positron emission and yet gain mass?

It describes that the final binding energy of the nucleus increases, thus making it possible by becoming more stable. But what actually causes the reaction to go in the first place? It's like saying this ball will go down the hill because it will lose energy - what gives it the nudge required? Is it something like energy from external gamma rays or something?