His going had all the earmarks of flight.
Well, then, why had the last horse died down the canyon?
Casey decided that he would go and see, though he was not hankering for exercise that day.

He took a long drink of water, somewhat shamefacedly filled a new canteen that lay on a pile of odds and ends near the tent door, and started down the canyon.

It couldn't be far, but he might want a drink before he got back, and Casey had had enough of thirst.
He was not long in finding the horse that had died, and in fact all the horses that had died.

There had been four, and the manner of their death was not in the least mysterious.

They had been staked out to graze in a luxurious patch of loco weed, which is reason enough why any horse should die.
Of course, no man save an unmitigated tenderfoot would picket a horse on loco, which looks very much like wild peavine and is known the West over as the deadliest weed that grows.