I don't mean on your farm, I mean in the general history of cattle evolution over the years, since the dawn of (cow) time. It may be bred into them to calve in poor weather, as an extra step of having only the strong survive, to both survive the weather and give them a bit of extra time to get their legs, when predators aren't out on the hunt. And then they also hide them in good weather, during the day and then stay with them at night when animals are out looking for food. We push them into calving in crap winter weather, but even the ones that calve in the summer, a lot tend to be in rain storms, or unsettled weather of some sort.
This is just my own thinking. I've never read it, or heard it elsewhere, but I have spent time thinking it over.