Android : Code verification: activity being closed

I have an activity where the user has to enter a verification code from a message.

The problem is that when I am minimizing my application to read the message and then when I open my application again the activity vanishes and I am redirected to the activity that was opened before it.

Why is this happening and where am I going wrong?

P.S. I cannot attach any code since I don't see what code I can attach to illustrate the problem.