Incidental Pre-Grieving
The way I usually explain pre-grieving is as an intentional act. But it often happens as just a side-effect of other things, such as:
You have a well-developed imagination and empathy. You read/hear/watch stories, real or fictional. You swap in yourself, or people you care about, as you relate and process - and bam! suddenly you're faced with a real emotional reaction to the loss of someone or something loved or important.
You figure out, for example as an aspiring actor, that you can cry on cue by imagining something that naturally brings tears.
You are abused, betrayed, let down, and so on, too many times. You grieve not exactly the person themselves, but some trait you thought they had, the person you hoped for or wanted or needed, the person they were only some of the time... by the time they die, there's nothing left to grieve.