It's two digits, what could go wrong? Well, have you ever used a program or device where you have a number pad, but a separate button that tells it what the numbers mean? For example, some oven timers let you punch in "20," but you have another button that tells it whether you mean 20 hours, or 20 minutes, or 20 seconds.
Or, if you ever used Photoshop, you have a place where you can punch in a number to make the image a certain size, but there's a separate box that tells the program whether you meant inches or pixels or percentage. So you type "300," because you want it to be 300 pixels wide, but you had the wrong measurement selected, so it makes the image 300 inches wide and your monitor explodes.
This was like that. When they typed in "-3.3," the autopilot happened to be set in the wrong mode -- feet per minute, instead of degrees. So those digits were interpreted as a descent rate of 3,300 feet per minute -- over four times faster than they intended. By the time the pilots noticed the error, they had a huge mountain coming their way pretty fast.
The thing is, the autopilot display screen gave no hints as to what mode they were using: Since it was just a two-digit screen, it showed the same "33" it would have shown had they been using the non-deadly mode. Even alarm clocks have more safeguards than that to let you know if you're accidentally setting the wake-up time for 6:00 p.m.
Which only rarely results in 87 casualties.