In reply to Howard1:
Because of the weather basically. Last summer was incredibly wet after a dry winter, so the grass was late to start growing then had it's roots waterlogged all summer and into early winter. It then turned very cold, damaging the grass further, and the grass didn't really start to grow until april so the buttercups had a real headstart. Most of the fields with an abundance of buttercups will have been grazed by cattle/horses last summer, and so they will have been poached too, again allowing the buttercups to take hold.
I have never seen so many buttercups in my fields, am hoping to get them topped this week as they aren't great for horses, although they don't seem to eat them, they can cause skin irritations. I think the fields will need spraying for next summer though. Don't worry, there are plenty of buttercups in the hayfields too, which won't cause any problems and so the wildlife will be happy!