I have gone through the responses because, it was a question I had as well.
I have come to the conclusion, until it can be explained better, that this is a phenomenon associated with dying. I would go further to say, that, this is a sign post of active dying.
I was prompted to research the last time, I saw my mother who was unresponsive at the time, a tear fell from her left eye. I wiped it and said to her, I will come to give you a bath in the morning. The neighbouring patients told me that within 5 minutes of my departure, my mother died.
When that tear fell, I was telling her all who send their love and that I was going to be there for her and expressed my love for her.
She knew I wasn't co.ing back to see her alive. I just wished she told me that she was leaving.
Having read these posts, in future. I will look at the tear as a signpost.