Oh Ghostwind, I am so so sorry for your loss. I wish I had a thought as to why, that's something we've been lucky with. I can't remember having a mare abort, we've lost a few foals and a couple of dams over the years but it was after foaling.
Do know someone locally that had a mare abort this year, they sent her to another place that had more pasture that needed grazed, there were black walnut trees in the back of the pasture :(
Sending big hugs.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.