I am sorry for your loss. We had an abort that was horrible years ago. I would recommend you taking your mare to your vet for a culture. I would suspect an infection before a dog chasing problem. Pregnant mares are pretty hardy when it comes to running and doing activities. Sometimes a mare can have had an infection that was never cleared properly (either from contamination or confirmation) and it makes carrying a foal to full term hard. Or she could have one now and needs attention. Her health and mind at this point would be of major concern.
Good Luck
Good Luck