Because there are four different types of black holes, there is no single cause for these phenomena. Basically, a black hole is a region of space where the gravitational field has become so strong that virtually nothing can escape from its grasp, not even the photons that produce what we call light. Black holes are effectively a rip in the fabric of space-time, caused by their intense gravity and thus all the known laws of physics – macro and quantum – break down. This is why we cannot know what happens to any matter that crosses the event horizon (point of no escape from the gravitational pull) of a black hole, until we learn those physical rules which apply to and inside this kind of spatial and temporal anomaly.
There is a theoretical construct called Hawking radiation, named after Dr. Stephen Hawking, the world renowned physicist and cosmologist from Great Britain, which actually escapes a black hole. It is a type of heat known as Black Body Radiation, and the amount emitted by a black hole is directly proportional to the gravitational field on the surface of the black hole. This means that the smaller the black hole, the greater the escape of Hawking Radiation. It is thought that the escape of this heat, together with the lack of new matter entering a black hole, eventually causes them to evaporate. While these properties (radiation and evaportaion) of a black hole have never been observed they have been verified by complex mathematical calculations.
The types of black holes are Supermassive, Intermediate Mass, Stellar Mass and Micro, and each form by a different process.
Supermassive black holes lie at the center of most, if not all galaxies, including out own, and are believed to form from the collapse of enormous amounts of matter and the merger of many smaller black holes at the rotational center of forming galaxies. It is around these objects that galaxies rotate.
Intermediate Mass black holes form by the collision of black holes, possibly from those in the center of forming galaxies, but they do not become the active galactic nuclei which is the fate of their Supermassive cousins.
Stellar Mass black holes form from the collapse of stars that are about 1 1/2 times the mass of our Sun, up to about 20 times its mass, or from the collision of binary neutron stars. In either event these are results of the deaths of such stars after they have exhausted all of their fuel sources. Stars that are not at least 1.5 times the mass of our Sun, will not form black holes, but will die by exploding in a nova or supernova and leave behind either a neutron star or a white dwarf.
Micro black holes are thought to have formed at the moment of the Big Bang and also from the collision of very high energy particles with each other or cosmic rays. While the other three types of black holes have been detected by indirect observational techniques and verified by mathematical equations, micro black holes are still just theoretical and await validation by experiment or other proof.