How to deal with Job security

How you deal with job security. My current organization had 120 employees when I joined 2 years back. Being a freshman I was worried about probation. Then I worked hard to overcome this hurdle. After 1.5 years the manager left the job. And suddenly people started leaving too. though I am happy for people who left on their own. But the organization has started asking people to leave all of a sudden. And that has made me think about my job security.