Another way to look at it is that nuclei have excited states just like atoms do.
You can excite a nucleus to a higher state where the neutrons an protons vibrate more. Often the most efficient way for it to release the energy is not necessarily via photon but via an interaction through the strong force, whereby an alpha particle it emitted. (interaction via the weak nuclear force are far less likely, by far I mean 20 orders of magnitude less likely)
So where the Boron and proton overlap and have a total energy greater than the rest mass of the carbon 12 they combine resulting in an excited C-12 state. The excess is released, on average, as with any radioactive decay, by the most probable route. ie. the C-12 emits one alpha, then shortly after the Be-8 left splits again into to more alphas.