Everything comes with its reason.

i think it means "bad thing happens" here. Just like you kick a football when it left your feet you happens to find someone walk in front of you far away. you are afraid of it. But the ball just fall on that person's head.

对。正解

每一件事肯定有原因, 是吧？

每一桩事，每一件事，都并非（不是）偶然的。