That is a pretty neet chart! I suspect that the 'island of decay' is just a manifestation of some kind of resonance in the nucleus that favors decay. I'm not sure what the interaction is (I'm not a physicist) but it seems to me that there is a nuclear condition that particularly favors lower energy states--this is the origin of the 'instability.' Nucleonics in some ways mimics some of the complex interactions of electron shell orbitals in chemistry with odd little resonances playing a part in things like 'Hydrogen Bonding" which is a misnomer because it involves the electrons and it isn't a bond because the particular electron isn't acutally shared. But interactions of the electric fields allows the electron to settle in a different set of 'preferred' orbits which induces a slight charge polarization--which is the real mechanism behind 'hydrogen bonding' which is a critical concept to ordinary things like protein folding, DNA replication, boiling water, etc. The point is that some of these resonances are very subtle but the results can have huge implications.

I think that the colouring is somewhat deceptive -- as far as the "nuclear world" is concerned, anything with a half-life longer that about 0.001 second is essentially stable.

Just because those "longer" half-lives turn out to appear rather short in our "macro world," is likely more of a chance thing, than anything else....