Honor culture, derived from the culture of the Scottish Highlands where the honor of oneself and one's clan were of paramount importance, was the main reason. You can still see echoes of this today even though it isn't as overt.
The Borders, not the Highlands. The former were far more violent and lawless than the latter, and where most of the Scottish settlers in Ulster were transplanted from. They were also Presbyterian rather than the Catholic and Gaelic-speaking Highlanders.
They scooped up plenty of English from the Borders too as I've learned from genealogy. Though just how Scottish or English the people along the Border considered themselves is a bit vague too (clans were all that mattered) Just adds another layer to the absurdity of the US term Scotch-Irish
