They could very well be playing; we have a 5-year-old Gelding who does that with the older Appaloosa gelding that's his "brother figure." If the ears are pinned it could be a sign that they're establishing the pecking order.
I also agree that halters in the pasture are dangerous. But, then again, they're dangerous even when the horse is tied. My mare spooked because of a drain hose someone was carrying and pulled back, breaking her halter. When she did so, the clasp under her chin (that rests on her cheek) skinned part of her face. The result was over 30 stitches, two weeks off of working, and a lot of treatment for proud flesh.
Horses should always be monitored with a halter on, and not left to free-pasturing. At the livery we rotate the horses into a pen to eat; they're in here for about 10 minutes and someone is always in the back watching the stalls and pens to make sure that the horses don't get into some sort of mischief.
If you've noticed, when horses play they don't "connect." Our playful geldings will buck up, kick out, and "bite" (just scraping each other with their teeth or lipping at one another), but it's never aggressive -- they don't hurt each other. And when it does hurt, the "victim" is often quite content in telling the "assailant" that it did hurt. Just today I watched my "elderly" gelding, Ben, playing with a younger gelding, Oreo. They "bit" at each other's faces, Oreo would rear up a little bit, and then they'd prance around and kick up a little bit. Then it was like "whatever," and they moved on.
If they're turned out overnight I would make sure to especially take the halters off; as many people have said, they pose a danger to the animal's well-being, which is further founded by Creampuff's story.
Personally, I would say that the most dangerous thing is the halter, not the behavior.
"Adapt or perish, now as ever, is nature's inexorable imperative." (H.G. Wells)
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