In what way is Romeo accidently responsible for Mercutio's death?

I think Romeo's choosing to keep his secret about why he
won't fight with Tybalt is most at fault for Mercutio's death. Romeo makes a great point
to Tybalt that he loves him, but never gets out WHY! This would have helped considerably
under the circumstances.

This appeared to Mercutio as
weakness in Romeo and he went after Tybalt then in defense of his friend, Romeo. Tybalt
wasn't about to take that and it resulted in a cheap shot to
Mercutio.

Obviously Romeo never intended for this to
happen, but he certainly had the ability to have prevented the situation by just letting
everyone know there is a new relationship brewing that would drive the families
together.