Likely natural causes, i imagine the survival of 1st and 2nd year birds is pretty low.
I thought it a bit of an odd release area as the coast across from the isle is well populated, would have thought Devon and Cornwall or Wales would have had some suitable habitats but I expect they looked into the viability of the area. I'll hopefully will get some sightings at some point, would be a life tick for me.
When i think back to when i was a lad some of what have become almost mundane birds of today would have been unbelievable back then, things like spoonbills, Glossy ibis, Cranes, Red K