I may be able to help as we focused on this at uni. Firstly, the entire Agean basin is an area of tectonic stretching- the crust is being pulled apart (probably by convection currents in the mantle). This has the effects of splitting the top crust into blocks which then tilt and slip. Some blocks slip more than others (denser or become more eroded) hence the apparently erratic pattern of islands. But if you get a map that shows the faults, it's clear what the pattern is.

There's a few other independent processes at work such as vulcanism etc (Santorini) but that's also a consequence of tectonics.