Why did the union troops in the feild of battle begin running toward the reserves?

They are running out of terror. They are desoriented and simply begin running. Check out the quote from GradeSaver,

"As the men watch the haze more, men start to run out of it. The defeated regiment runs through the young troops. The "blue line" only watches them go. The officers try to stop their flight, but the other troops only watch. They are still "blue" and considering an outside action. War, though so close to them, has still not touched them. Though Henry can observe what "the struggle in the smoke" has done to other men (made them wild and flow like a flood), he can still only think about this. He is resolved to view this beast of battle, and only that he might run. He still does not know. He will find out in the next chapter."