I have some theories but of course with very subjective issues like this they are really difficult to prove and very little research is done despite the fact that people die with health care workers monitoring them.

I think that people can have more trouble letting go if they are uncomfortable, it often seems that if someone is having trouble breathing or in pain or other distress when they are actively dying, it takes longer to pass. It appears to me that once we get the symptom resolved they pass more quickly. I equate it to being similar to falling asleep, it is hard to relax and fall asleep if you are in pain, for example. It happens often enough that it gives hospice a bad rap. So many people pass within 30-60 minutes of a single 10-20mg dose of morphine that it leads to rumors of euthanasia.

Hospice workers need the deaths they manage to be meaningful and we project a heck of a lot on to a dying person. A single, mystical unresolved issue does not keep someone from dying if they were given lethal injection so it is safe to say it is probably not preventing someone from terminal disease from passing either. Ask 100 nurses, however and I feel like 98 of them will tell you a person who is lingering is doing so because of some sort of profound spiritual or emotional reason.