Stem's Trigger was a culmination from a lifetime of being compelled into different roles, which they excelled in at the expense of all else. The star dancer who neglected their studies; the athlete without a social life; the friend who helps their friend through a hard time, but loses their job and romantic relationship in the process; the family-provider who never sees their kids.
Oftentimes, these extreme overcommitments were not *asked* of them explicitly, but feeling they were expected to; circumstance requiring someone to step up and there being nobody else to do it.
Each time they excelled in something, another aspect of their life blew up in some way. This was manageable as a teenager, but as they got further into adulthood, the collapses became more and more catastrophic and harder to come back from. Your high school breakup is easier to manage than your divorce in your 30s. They spent a year of their life taking care of a terminally ill parent with whom they had a contentious relationship, and when they passed, Stem realized they had nobody close enough to them to help them through the mourning process.