According to Annie Proulx's book, it was not yet August when Aguirre rode up with the news about Jack's Uncle Harold having pneumonia.
It was the mddle of August when Aguirre said to bring the sheep down from the mountain.
So, there is absolutely no connection between Uncle Harold's illness and the sheep herding job ending in the middle of August instead of September, the "official" end of the sheep grazing season in the mountains.
It was a coming snow storm, probably a blizzard, that caused Aguirre to decide to end the boys' job a month early.
When Aguirre went back the 2nd time to tell Jack that his uncle recovered, he gave Jack a bold stare. Annie Proulx does not explain that; but, I think that he was remembering what he had seen with the binoculars when he first came up with the news of the sick uncle. Aguirre could have put a stop to them goofing off by riding up while they were having sex out in the open.
I don't think Aguirre was even bothered by his shepherds having sexual activity; but, it bother him that they goof off too much with lots of horseplay and rough-housing while letting the dogs babysit the sheep. Their acting like the immature teenagers which they were when they should have been working as responsible adults was to what Aguirre meant when he accused them of "stemming the rose." The expression comes from removing the stems from roses, leaving only the rose blossom and there being no way too keep the rose looking good. "Stemming the rose" has to do with wasting time when there is work to be done.
In regard to Toast's last two questions here:
What would have happened between Jack and Ennis if Aguirre had not ended the Brokeback work as early?
If Jack and Ennis had been together for another 20 or 30 days, what might have happened?Oh, there are lots of unanswered questions which we as the reader of the story or the viewer of the movie can bring up.
I would say that if they had worked the full season on Brokeback Mountain and Ennis had his truck parked at Signal in Aguirre's parking lot just like Jack's was, Ennis might have changed his mind about getting married to Alma in December 1963. It certainly was not a "shot-gun" wedding because Alma did not get pregnant until January 1964.
He might have written Alma a "Dear Jane" POSTCARD, to tell her the marriage was off. In the book, Ennis's brother, K.E. lived in Signal when Ennis went to work for Aguirre. Alma might have lived in town or on a ranch there.
After the guys got paid by Aguirre, they "ate . . . at that place in Dubois" in the book. After the meal, they drove their trucks, which had been parked on the street, off in opposite directions.
Two quotes from the book related to the 1963 departure scene and how Ennis talked about it in the Motel Siesta in Riverton in 1967.
"You goin a do this next summer?" said Jack to Ennis in the street, one leg already up in his green pickup. The wind was gusting hard and cold.
"Maybe not." A dust plume rose and hazed the air with fine grit and he squinted against it. "Like I said, Alma and me's gettin married in December. Try to get somethin on a ranch. You?" He looked away from Jack's jaw, bruised blue from the hard punch Ennis had thrown him on the last day.
"If nothin better comes along. Thought some about going back up to my daddy's place, give him a hand over the winter, then maybe head out for Texas in the spring. If the draft don't get me."
"Well, see you around, I guess." The wind tumbled an empty feed bag down the street until it fetched up under his truck.
"Right," said Jack, and they shook hands, hit each other on the shoulder, then there was forty feet of distance between them and nothing to do but drive away in opposite directions. Within a mile Ennis felt like someone was pulling his guts out hand over hand a yard at a time. He stopped at the side of the road and, in the whirling new snow, tried to puke but nothing came up. He felt about as bad as he ever had and it took a long time for the feeling to wear off.
"That summer," said Ennis. "When we split up after we got paid out I had gut cramps so bad I pulled over and tried to puke, thought I ate somethin bad at that place in Dubois. Took me about a year a figure out it was that I shouldn't a let you out a my sights. Too late then by a long, long while."