As indicated from Jim Conklin's "shaking whisper," of what was he afraid?

Jim was afraid the artillery wagons would run him over,

[Suddenly, as the two friends marched on, the tall soldier seemed to be overcome by a tremor. His face turned to a semblance of gray paste. He clutched the youth's arm and looked all about him, as if dreading to be overheard. Then he began to speak in a shaking whisper: "I tell yeh what I'm 'fraid of, Henry--I'll tell yeh what I'm 'fraid of. I 'm 'fraid I 'll fall down--an' them yeh know - them damned artillery wagons--they like as not 'll run over me. That 's what I 'm 'fraid of--" ]