I was spelunking over the map, looking for places I hadn't visited yet on this character. To my surprise, even though I had cleared Lost Valley Redoubt (with Bard's Leap) I hadn't been up to Cradle Stone Tower. After dealing with that, I could see a Nordic ruin to the west (Valthume), I just had to carefully descend a steep rocky cliff.
As I started down, I heard the roar of a troll. I looked around, but at first I could see nothing. Then the troll appeared from his shelter at the base of the cliff, a little north of me. It wasn't me that he was in a huff about. In fact, he started west up the slope in the general direction of Valthume, stopping to roar again and make imperative gestures. I couldn't see what had gotten his goat, then I spied the other troll. This was just after sundown, so they both blended in with the terrain.
Those two bellowed and gesticulated at each other for some time before getting into a fight. This took them around the arm of rocks and down the slope to the north. Of course, they were evenly matched, so the contest was indecisive. From my lofty perch above them I used my Storm shout on them. Troll zapping is fun! I've seen trolls together before, but never antagonistic toward each other.
Another weird thing today, also in the Reach:
I saw a small Forsworn camp up on a hill. As I crept up for a closer look, I heard someone say, "You made a big mistake coming here!" and sounds of a fight. At first I thought bandits were attacking, but no, it was a pair of Stormcloak soldiers. They wiped out three Forsworn like it was nothing. I'm sure I've never seen such a thing. They just stood in the camp, one of them munching on a loaf of bread, lol.