The bacteria that causes rain rot is present just about everywhere. It's opportunistic, when the conditions are right it strikes. Most often towards the end of winter. The vitamin A doses help to boost the immune system so the horse never gets infected or is able to fight it off.
The moment you cut grass and start drying it vitamin A is leeching out into parts unknown. It breaks down fairly rapidly. Towards the end of winter you have hay that has been stored for a while. In the summer you have fresh cuttings. The long coat of late winter helps hold in heat and moisture and dirt. Perfect for the bacteria to grow.
Vitamin A for Rain Rot?
rain rot/scald possibly