To understand the formation and cause of black holes, we must first understand some of the theories that surround them. It does little good, however, to become too technical or the information is useless to the average person. This is too often a problem in science today, and is why many people have difficulties when learning or grasping science.
Four forces; the strong nuclear force, weak nuclear force, gravitation, and electro-magnetic force govern the universe of today. Black hole physics deals mostly with gravitation, though the other forces do have importance.
With gravity, every body that has a mass exerts an attractive force on every other body that has mass. In other words, two masses are drawn toward each other.
A star is formed when gases and other material collect under the force of gravity, becoming more and more compact until atoms are forced together, creating nuclear fusion. Nuclear forces (collectively) resist atoms coming together, but at the point of the birth of the star, the power of the nuclear forces to repel is overcome by the power of gravity to attract.
Temperatures rise until the atoms are ionized, meaning that they are stripped of electrons. As basically as possible, four hydrogen atoms are ionized and forced together, creating two helium atoms. The helium has slightly less mass, and the excess mass is released as photons of energy.
The star achieves equilibrium. That is, the forces of the radiation produced by the fusion balances the forces of gravitation. One pushes outward, the other pulls inward, but they are balanced so except for fluctuations, the star becomes relatively stable.
It doesn't remain stable. It takes more energy to combine helium into more dense atoms, but the hydrogen is being consumed in enormous amounts. Eventually, hydrogen burning gives way to helium burning, to carbon burning, and so forth. Each time a new level is reached, the outward force becomes greater than the inward force, yet the number of atoms decreases since it takes more of the simpler atoms to make the more complex atoms. The star swells.
For example, when our sun begins to burn primarily helium, it will expand to well beyond the orbit of Venus and could easily encompass the earth.
Eventually, this means that the star will get to the point where there is a big difference between forces bringing atoms together and those forcing them out. The star then forces off the outer layers in a vast explosion. Relatively minor explosions, which our own sun is slated to because of its gravitational pull and mass, are novas.
A star after a nova then collapses again, much more drastically, due to the density of the core. The collapse only stops when the other forces prevent further collapse. At this point (the ultimate fate of our sun), electrons, with a negative charge, have been forced into protons, with a positive charge, to produce neutrons, with no charge at all.
Since most of the space in an atom is just that, space, this means that the core of the star contracts enormously. Our sun, a thousand times bigger than the earth, will become smaller than our moon. A neutron star is born. (Oversimplification, our sun will become a white dwarf, not a neutron star, but the principle is the same. It takes a super nova to produce a neutron star.) A single teaspoon of the neutron star would weigh many hundreds of thousands of tons.
Not all stars are created equal, though. Ours is a small star. There are many stars that are far more massive than our sun. They burn their hydrogen, then helium fuel far faster. More importantly, they have a much greater gravitational pull because of their mass.
The explosion of the outer layers of the star only release the lighter substances, and the gravitational pulls are even greater on what remains. At a certain point/size, the other forces can no longer prevent the collapse caused by gravity. All remaining matter within the gravitational pull of the collapsing star is compacted further and further until it becomes a black hole.
Put in a different way, the entire mass of the star is compressed into an area far smaller than a period on this page, yet it still has a gravitational pull as strong as the original star. At this point, at a certain distance from the spot (known as a singularity with the distances away from the singularity known as the event horizon), and which can be determined mathematically according to the mass of the star, the gravitational pull is so strong that photons of light cannot escape the pull of gravitation.
This is a black hole. Anything passing into that region is compressed and merely added to the singularity, increasing its mass and gravitational pull.
Astronomers now know many black holes, indirectly, including two super massive black holes near the center of our galaxy, each with the mass of several millions of stars the size of our sun.
The reason for saying 'indirectly' is because since light cannot escape a black hole, we must rely on other methods to detect them. For instance, material spiraling in will give up huge amounts of energy, and that energy can be detected up until they cross the threshold or event horizon.
The very simplest answer to what causes black holes is that the force of gravity causes them.