Riddle: What is so fragile that saying its name breaks it? Think, think, think.
We use the adjective “fragile” for things (and also people) who can get hurt or broken easily. Fragile things are delicate and tender, therefore we need to be careful when dealing with such delicate concepts. And silence is one of them. When we say silence or silent, we mean that there is not a single sound. No talking, no music, no horn…
So when we say the word “silence” it just destroys that quiet moment and ends it. It’s just like the famous game called “1-2-3 Hush Puppy”. You need to keep quiet as long as possible and the one who makes a noise breaks the silence and loses the game.