This is a good example of what emerges from sufficiently complex systems.
As a system becomes more complicated, new properties begin to emerge.
"The ability to reduce everything to simple fundamental laws does not imply the ability to start from those laws and reconstruct the universe..The constructionist hypothesis breaks down when confronted with the twin difficulties of scale and complexity. At each level of complexity entirely new properties appear. Psychology is not applied biology, nor is biology applied chemistry. We can now see that the whole becomes not merely more, but very different from the sum of its parts."
It's not so much that no one, if they'd sat down and thought about it, could have predicted that people would make music with old computer parts. People make music with everything they can get their hands on. It is that computer parts would not have existed without a certain level of system complexity; they simply require too much infrastructure to be available at a lower level of complexity.
There is no inherent difference between a person turning an old trash can into an instrument and a person turning an old computer into an instrument. However, one of them only requires that trash cans exist, while the other requires that computers exist.
We will always be able to hit something, or pluck something, and get a noise out of it. With a certain amount of experimentation and elbow grease we could figure out how to get a musical scale out of just about anything. However, new tools require us to learn new techniques. Notice that no one was banging on the old computer parts. The concept of "hit it until it makes a sound you like" is not new, and might even be buried somewhere in our genetic code. Taking over an old computer system's drive unit and figuring out how much juice to give it, and for how long, and in what combination, to get a particular sound requires an entirely new thought process.
Not everyone likes every instrument. If you happen to want to play an instrument that does not exist you have two options: invent it or wait for someone else to invent it. To invent an instrument requires knowledge, which means that someone might learn something new just so that they can invent (or play) an instrument.
Once these new skills and ways of thinking are out there they can be applied to areas where they might not have been generated spontaneously and they can even inspire brand new ideas. Thus increasing the complexity of the system even more and allowing for even more emergence.