Please explain "Why did the wedding guest beat his breast?"

The bride had entered, the minstrels were going on and the wedding ceremony was to begin in the hall. But the ancient mariner was holding the bride-groom in a magical spell of poetry, telling him the story of the sin in the ghost ship. So only it was normal that the wedding-guest beat his breast fearing that the ceremony would be over without him. The anxiety and suspense Coleridge created in this part of the poem was only a prelude to the unbelievable things he was going to tell in the poem.