Question

how the alpha radiation accours

Answer 1

because it becomes more stable by emitting alpha particle (or any other kind of decay).

Nuclei become more stable once their proton & neutron count is close to "stability islands" which are just "magic" numbers that atoms are more stable when their proton & neutron number gets close to them.

The reason behind why these islands exists can't be explained easily, and to some extent they arise from different theoretical models of the nucleus, which in general are almost impossible to be modeled accurately (this info is kinda outdated and from an old book, maybe today with fast computers the situation is different).