The answer is that innumerable circumstances can trigger this process, there really is no one answer.
Spiritual awakenings can happen at any period in your life. They can be spontaneous, but they can also be triggered by major life changes such as: illnesses, tragedies, and traumas such as life-threatening illnesses, car accidents, divorces, war, midlife crises, and much more.
Awakening can also happen due to your temperament. For instance, you might be an old soul or a sensitive empath by nature. Furthermore, for some people, spiritual awakenings are even triggered by deep but challenging twin flame relationships.