We can all guess - Lepto, Neospora, moldy hay, twins and you only found one, cow bred early after parturition with a lo-grade uterine infection, possibly something wrong with the fetus, E.G. AM,NH, some other genetic thing.
Hope you don't get more, take the next one in to the vet for sure, take the placenta too, draw a blood sample from the one that aborted now and again in 2 wks and see what changed, a good way to test for diseases that cause abortion. Most times it's hard to find the cause. Asking your vet what is common in your neck of the woods is a great start.