LOL Probably several reasons for that. They are very people oriented, and their owners visit with treats in their pockets,or their owners let them get away with murder, such as some horse owners let their horses push their heads against them and rub them knocking them almost off their feet. My guys are friendly, but I would never allow them to use me as a scratching post!
What are you going to be doing in the pasture? Perhaps you can just put a mound of hay out before you work and sprinkle the mound with a little bit of grain and they will be preoccupied with other things while you are out there. I would also invest in a riding crop and keep it with you at all times- use it as an extension of your arm and if a horse is getting to close, pivot sharply- hold out the crop (don't smack the horse) and tell the horse to hold. But if you do this, make sure you are no where near the back feet, because a horse can kick and reach a long way. I carry a crop with me all the time, because my horses get a bit to nosey and I use it to establish the fact that when I am in the pasture, I AM ALPHA they are not. Again, I do not smack them or hit them unless they are really acting out, I use it to make my arm longer and to establish the space I need to work safely.