One of the horses that I ride is like that. Every single time,he'll drop a half a foot away from you when you release him, roll, and then walk to the gate and lick it. ALL the horses have to lick the tree by the gate when they come out. I don't know if they're missing minerals in their diet, or if it's monkey see monkey do. But with the tree, no one touches it going back into the pasture, only coming out of it.
hhmmm is it maple? Maybe the tree is sweet?