Ask HN: Why did this shut the entire network down?

When I was in High School I was using the computer lab and noticed an ethernet cable hanging down next to the switch (I think had about ~24 inputs). I plugged it in and a couple seconds later the internet for my entire school was down. Not a single computer in the school had internet. The head of IT ran into the room and had no idea what was going on. I unplugged it once I realized that it was my fault the network was down and everything was back to normal almost immediately. So my question is: Why did plugging the cord in shut down the internet for the entire school? (I assume that it was already plugged in to the switch and was plugged in again, but it was so messy I couldn't confirm that)