Horse itching on tree?

Today I put my horse in a different pen from her normal one (down the road) to one behind my house. I've been doing this for about a week, to let her graze with two other horses who the neighbors own. Today I noticed her scratching her neck, sides, and rump on a small pine tree in the pen. I walked her away from it because she was scratching up her neck, but she literally ran back and started rubbing again. Ive never seen her do this before, is there something cause it, or is she just bored?