At its root, it was because early computer programs (and some operating systems) stored years as a two-digit field. So the worry was about some long-forgotten piece of code suddenly encountering a condition it couldn't cope with and shutting down. The obvious implications were for things like payroll and time-tracking systems, but there was a small, but legitimate, fear that the computer problem could affect the systems that run the power grid or banking networks.

In fact—as real experts knew at the time—the risk was very tiny and had mostly been ferreted out months ahead of time. But hysteria doesn't easily yield to rational explanation, and no one could say there was a nonzero chance of problems.

I'm sorry, but we have a 20 year rule, so anything past 1994 is not allowed here.