All black holes are caused by gravitational collapse. Such collapse can be best understood at the stellar level. However, there is nothing which contradicts the possibility that black holes could be of any size, from hundreds of thousands of stellar masses to subatomic.
The most likely known-universe source for black holes are stars over the Tolman-Oppenheimer-Volkoff (TOV) limit in mass (estimated to be between 1.5 and 3 solar masses). A mature star normally exists in an equilibrium between the outward force of plasma dynamics and nuclear hydrogen fusion and the inward gravitational force of its own mass. (Plasma is the fourth form of matter, formed when atoms are stripped of their electrons.) In the late stages of their lives, such stars may no longer have sufficient hydrogen to sustain the optimal hydrogen fusion, and is forced to far less efficient forms of fusion which are ultimately the source of all 92 known natural elements. Such a star is said to have left the ‘main sequence’. Eventually the energy from these less efficient forms of fusion will no longer suffice to balance gravitational contraction. However, as it begins to shrink in size, this contraction in volume itself forces plasma into such proximity that fusion energy comparable to the sun’s entire energy output is released in the space of a few seconds. For a large enough star, the result is a supernova.
A star which has gone supernova may blow off as much as 90 percent of its mass in the explosion, glowing matter that will be visible for centuries to come as an expanding nebula. One such documented event has occurred within historic times, resulting in the still-expanding Crab nebula, with a small, spinning pulsating object at its heart which is now commonly believed to be a neutron star. The remaining mass of this star was not over the TOV limit, although it did exceed the Chandra limit for white dwarfs.
An alternate scenario which produces a similar result might be when the star comes into contact with a great deal of excess matter without being able to raise its temperature to compensate. Such a condition could result as a result of a star making contact with an extra-stellar current or nebula consisting primarily of an element other than hydrogen. Alternately, it could come about as a consequence of a close encounter or even collision with another star.
If, after the supernova event, the star’s mass is still over the TOV limit, the force of gravitational collapse will be greater than the mutually repulsive neutron-neutron interaction, and then such a star is speculated to become a black hole.
