What causes a Phase-Transition?

In a very general picture a phase transition is a change of between different states of matter.

The cause for a phase transition is a change of a thermodynamical parameter, such as temperature, pressure, volume or magnetic field. The primary example is a change of temperature as the cause for a phase transition.

What exactly happens on a microscopic level depends on the details of the system, so there is no general statement possible.

The change between different states is so special because several things might happen:

• Change of symmetry (e.g. from continous to discrete)
• Latent heat
• Divergence of specific heat, susceptibility, ...

A good starting point if you are interested in all these phenomena is the Wikipedia article about phase transitions. The underlying theory is in development for several decades, where the works of Landau and Lifshitz provide a good coverage (Statistical Physics Part 1) or any modern introduction to Statistical Physics.