Horse Problem?

I have 2 quarter horses, 17 & 21. The 17 yr old is dominant. When I put blankets on them for the very first time, the 17 yr old began herding the 21 yr old, making him gallop around the field off and on for an hour or so, which he has never done prior to wearing the blanket. And when I took them off, he stopped herding the older horse. Is this due to the blankets? Or is he just expressing his dominance in a new way?