Positive feedback is a term used by scientists and engineers to describe a feedback loop process where the output of a system drives increased input to the system.
In human terms, one such example is drug addiction, where increased use leads to increased desire for the drug.
A more interesting example can be seen in the 'sudden acceleration' lawsuits against Audi some years ago. Once the cars were examined, it was determined that the drivers had been pushing on the accelerator pedal, rather than the brake. Because they were convinced that they were right, they pushed harder as they gained speed. In some cases, the drivers injured their own legs from the pressure, and bent the pedal.
The past 30 years have seen such a loop in the housing and financial sector. As house prices went up, they released fiat cash into the system, driving prices even higher. This, of course, led to the 'brilliant' idea of packaging mortgages. At some point, the profit margin became so huge that no 'real' industry could compete, which pulled even more investment capital into housing derivatives, and further crippled manufacturing.
Without a governor in place, by way of careful regulation, these crashes are inevitable. The capacity of the internet in moving money only sped things up a little.