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.