The dwarves are nervous because the Cephalrrior called them 'dwarves' and there are only two groups that the dwarves have come in contact with and that is the Equestrians and the Reapers. Since the Equestrians called them 'new friends' the dwarves assumed that the Equestrians have never spoken to the Cephalrrior before, so this makes the dwarves believe that the Cephalrrior are the Reapers.