Defensive termination is when someone is fired from their job to prevent something bad from happening. Just like how mommy and daddy might take away your toys if you're being too rough with them, sometimes bosses have to fire people if they think they might do something really bad.
For example, let's say someone at work makes a lot of mistakes and their boss is worried that they might make a big mistake that could hurt someone or cost the company a lot of money. In that case, the boss might decide to let that person go before something bad happens.
Even though it might seem mean to fire someone, sometimes it's necessary to protect everyone else from harm. Just like how mommy and daddy might take away your toys to protect you and your friends from getting hurt.