Why am I being prompted to Authenticate when Signing?

As a Signer, you might be asked to authenticate when you open documents based on the sender’s account requirements. In some cases, senders only require the authentication once, but the sender can require the signer to authenticate on subsequent document access attempts.

If you change devices or browsers you will be asked to authenticate again.
 

Reference:
May 2016 Service Pack Notes
Why are signers being prompted to Authenticate?