What causes speciation and why might there be very unique species on an island?

Geographic isolation and reduction of gene flow are the main causes of speciation.

A very unique species might evolve on an island because the environment and living conditions are different and it will need to adapt to it. (e. g. island dwarfism)

Explanation:

Geographic isolation most often occurs with populations that are completely separated by a physical barrier, such as a mountain range, river, or desert. The separated populations adapt to their own unique environments, becoming so genetically different from one another that members of one population cannot breed with members of the other.