IMHO:
It wasn't until the confrontation, and Jack said "I wish I knew how to quit you." that's the dam in Ennis that broke. We've seen time and time what was going with between the two, Jack's trying over the years to convince Ennis to build a life with him, Jack said it perfectly that final day, he found his voice and if Ennis would kill him or quit him was chance Jack took. And what provoke this whole thing, Jack felt Ennis was pushing further away.
What was real motive for Ennis doing this, after a night of sefl-confession, intimate moment between them, older men now, supposedly secure in their love for one another, why in heck did Ennis think he could jerk Jack around one more time. It's like saying to Jack, "What's going in my life, will always be paramount to your needs." Using the same premise he uses years earlier after the reunion, there is something greater than you. Come on, Ennis did it because Ennis felt he could do it. Of all people to one who loyal and loving.
Losing Alma, his indifference towards Cassie will never impact him as much as losing Jack, but Jack was suppose to understand when Ennis makes a demand thats it, and what Jack's feeling, what Jack need, and his wishes have to be defferred because Ennis needs out weighs Jack.
Ennis need for control was slipping, when he realized that Jack was thinking, wishing there was a way he could get away from him. Why do you think he wrote him, to find out he was still coming in November? Just the idea of Jack not coming must have crossed his mind. We all know fate took care of that.
It was always fear with Ennis I'll give you that, but its also control. When you have fears you must have control.