Source:  Various

According to the BBC, researchers in the Czech Republic observing fertility loss among Zebras in zoos, found that pregnant females, when exposed to new males introduced into the herd, engaged in mating behavior which often resulted in abortion.  

They hypothesized that this behavior among equines avoided loss of newborn offspring since males in the wild will kill offspring of other stallions.  

To test the hypothesis among horses, they sent out questionnaires to horse breeders which revealed similar results.   Mares sent out to be bred with outside stallions often lost the pregnancy when they returned to the home farm which also housed male horses.  However, mares bred to home farm stallions rarely lost the pregnancy.

The abstract of the paper, by Ludek Bartos, et al, states:

Based on questionnaires from horse breeders, we found that bringing a pregnant mare which had been mated away from home into a vicinity of a familiar male who was not the father of her foetus increased probability of pregnancy disruption. These mares aborted in 31% of cases, while none of those mated within the home stable aborted. Repeated sexual activity either by a stallion or dominant gelding from the normal home group was observed shortly after the mare came from away-mating. Pregnant mares isolated from home males by a fence were even seen soliciting them over the fence. We speculate that, once returned to the home “herd”, and introduced to familiar males, mares were more likely to terminate their pregnancy to save energy and avoid likely future infanticidal loss of their progeny by dominant male(s) of the home social group. This is a newly discovered phenomenon where a mare manipulates the male’s paternity assessment by promiscuous mating. It may explain a common increased incidence of foetal loss in domestic horses occurring in nearly 40% of pregnancies. We conclude that the common practice of transporting the mare for mating and then bringing her back to an environment with males, stallions or geldings, which did not sire the foetus, is the main cause of high percentages of pregnancy disruption in domestic horses.