Is it mandatory to fill security questions and answers in order to download apps?
Whenever there is a change in account status, it blocks all subsequent Apple ID authentication from completing successfully until you resolve the issue. I've seen this happen with:


Security Questions / Answers when this rolled out a few months back.
Credit Card / Billing issues.
Purchases from a device previously unused by your account.


Only after resolving the particular issue will you be allowed to continue.