I'm reading Blood Meridian by Cormac McCarthy and am having trouble understanding what is happening during page 232 where the group is getting ready to sleep in a barn.

"No one moved. In that cold stable the shutting of the door may have evoked in some hearts other hostels and not of their choosing. The mare sniffed uneasily and the young colt stepped about. Then one by one they began to divest themselves of their outer clothes, the hide slickers and raw wool serapes and vests, and one by one they propagated about themselves a great crackling of sparks and each man was seen to wear a shroud of palest fire. Their arms aloft pulling at their clothes were luminous and each obscure soul was enveloped in audible shapes of light as if it had always been so. The mare at the far end of the stable snorted and shied at this luminosity in beings so endarkened and the little horse turned and hid his face in the web of his dam's flank.”

What is happening here and why do the horses seem so frightened?