Space isn't empty, and near-Earth orbit is downright crowded. Every month, some junk burns up during re-entry as ever more is introduced into orbit. Poking around NASA's Orbital Debris Quarterly archives reveals the story of space junk from deliberate to accidental, and all of it hazardous.
Prior to June 1961, the entire population of artificial objects in near-Earth orbit was just over 50 objects, all spacecraft and rocket bodies. Then the Ablestar launch vehicle deployed its payload, the Transit 4A satellite, and exploded just over an hour later. The explosion created nearly 300 debris fragments, over two-thirds of which were still in orbit in 2002. After that, space just got messier.
Anti-satellite testing caused a whole lot of mess as the Soviet Union and the United States took turns proving they could blow up their own satellites. Between 1968 and 1982, the former Soviet Union conducted 20 tests, creating somewhere over 700 catalogue debris fragments, 301 of which are still in orbit. In 1985, the United States tested its own system, producing a whiff of debris, none of which remains in orbit. Realizing that all these explosions were producing a terrific mess, by collective international agreement, no one conducted any more tests of anti-satellite systems that produced debris. This agreement held for 20 years.
In the late '80s and early '90s, a whole lot of people did a whole lot of talking, eventually agreeing to voluntarily reduce the amount of junk they were producing. It worked for a while, reducing the growth rate of new debris cluttering up near-Earth space from a fairly steep climb in 1968 through 1988 to a far flatter climb from 1992 to 2006.
A few years later was a far more spectacular explosion. In June 1996, an abandoned upper stage rocket "broke up," the orbital debris euphemism for "suddenly exploded." The abrupt fragmentation of the rocket stage produced 700 pieces of distinct debris. The stage was the Pegasus Hydrazine Auxiliary Propulsion System (HAPS) from the STEP II mission that had launched 2 years previously. The event produced an order of magnitude more debris than models suggested it should have, forcing NASA to rethink what they did with abandoned craft. Eventually they figured out that the explosion was enhanced by excess fuel, leading to a procedure change where spent stages perform a propellant depletion maneuver to both reduce fuel and to place objects in a decaying orbit where they will (hopefully) burn up on re-entering the Earth's atmosphere within five years.
Most tracked events are nowhere near that exuberant. The next month, the French CERISE spacecraft was pinged by a fragment of an Ariane 1 launch vehicle that had exploded a decade earlier. No new debris was created, the spacecraft recovered, and everything kept on whizzing about the planet.