In two large strides the Trunchbull was behind Eric's desk, and there she stood, a pillar of doom towering over the helpless boy. Eric glanced fearfully back over his shoulder at the monster. "I was right, wasn't I?" he murmured nervously. (13.85)
The Trunchbull seems to like it when she can scare people for maximum effect. She hasn't even touched Eric yet, but he knows she could hurt him at any moment. So of course he's fearful and nervous, not knowing when this pillar of doom will strike.