Our little one did the same thing at 6 months. I can't say how early it started as she joined our family through adoption at 6 months but she already had a raging bald-spot which suggests that it had been happening for a while! Now at 8 months she hardly does it at all and her bald-spot is recovering quite nicely indeed. I also freaked out when I first saw her do it as, to me, it looked like she was distressed but in hindsight it was just something she did and didn't seem to bother her in the slightest.