The rumbling, chasing and mounting, etc. are all dominance behaviors. They have established a hierarchy even if you can't easily see it. Some pairs have more noticeable conflict where it's easier to determine who the boss is, others are relatively harmonious and it can be hard for the humans to tell. Our original queen bee, Linney, was very obviously the boss because she was a bit of a tyrant and was very adamant about reinforcing the hierarchy. She lived with two consecutive pigs, Frenzy and Sundae, and she was very clearly the one in charge and the others had to wait their turn at the water bottle and regularly got shut down by Linney just so that they knew where they stood. Our current boss pig, Hadley, is just a lot more laid back and it's much harder to see who the boss is. Even if you can't see it, the pigs have arrived at an understanding!