She does this because she is slightly heard bound. As long it does not get any worse but her mind back to work. If it gets worse, the second she starts to get silly put her feet to work fast. Do lots of circles. Then let her walk again. Repeat if she keeps doing it. She will get it soon that when she calls out again she has to go to work fast. Then she will stop.
Horses are herd animals, and as a result she feels uneasy being alone. There's safety in numbers, and she's now alone with just you, a predator, on her back. You could perhaps do some ground work with her to help her trust you a bit more? Walking over plastic bags, and grids in hand are both going to bombproof her and convince her to trust you at the same time.
My mare used to do this a lot, I'd catch her in from the field (Where her herd are) and then take her out on a ride on her own, and she would call out to them. I just told her to move on, and did ground work with her, and it rarely happens now.
The herd instinct is exactly that, and instinct, and you'll never completely abolish it, but by improving your relationship with your horse and perseverance, you'll start to see improvement.
She does it because she's not paying enough attention to you. Make her work when she does that and put her head back in the game. Allowing her to stop and call will lead eventually to running back to the barn due to herd sourness. Nip it in the bud, now!