Could you please tell me what the relationship is between the phrase:
"How'd you manage that?"
and the next sentences:
"I felt stupid. I should have been able to do it, I thought. It’s a simple thing."
Here is the full text:
My back struck iron: the trailer’s wall. My feet snapped over my head and I continued my graceless plunge to the ground. The first fall was seven or eight feet, the second perhaps ten. I was relieved to taste dirt. I lay on my back for perhaps fifteen seconds before the engine growled to silence and I heard Dad’s heavy step. “What happened?” he said, kneeling next to me. “I fell out,” I wheezed. The wind had been knocked out of me, and there was a powerful throbbing in my back, as if I’d been cut in two. “How’d you manage that?” Dad said. His tone was sympathetic but disappointed. I felt stupid. I should have been able to do it, I thought. It’s a simple thing.