Join Date: Apr 2008
Location: Orange County, NC
It could be the treats, but I wouldn't jump to that conclusion after it happening just once and you not actually seeing what led up to it. Getting your attention is possible, also. Were you wearing something new or different that could have a familiar (e.g. Treat) or unfamiliar smell/look/etc? Horses are very curious and taking things in their mouth is a way to examine them.
All that said, don't over think it. You may never know the why, but you never want the behavior, so correct it like you did, move on, and don't let it affect your training plans.
On the sixth day, God created the Quarter Horse.
On the seventh day, he Painted the good ones.