When George lived alone for a time he came out to eat his grass without any fanfare but now he lives with a little sow and has two more next door and he rumbles everywhere he goes to show what a great big boar he is! When his petite cage mate first moved in she also had a little rumble back at him but she accepted him as dominant (a first for nervous George!) and now she just lets him get on with it. He rumbles into the snuggle tunnel to claim it and then rumbles around inside, and she'll just wait. When she sees her chance she shoots through at such speed and out the other side before he knows what's happened! Sometimes he blocks it with his bulk so she just clambers over the top prompting aggrieved rumbling from inside as he tries to retain his dignity! She could easily walk around - there's plenty of room - but she's cheeky - she's only been here 2 weeks!
Two boars decide who is the dominant pig by testing each other in subtle ways. It doesn't really matter which roles they take as long as they're both happy with it. If they're well bonded but go to a new home they'll be rumbling again as they get to know their new territory. You are right not to interfere as the rumbling is just them working it out. It might settle or they might spend their lives rumbling away on and off as part of normal conversation... I think George will be a bit like that now but we'll see over the next few weeks. It will be better then the profound silence of a lonely pig. If there is a real fight and blood is drawn this is not a good result - the bond is broken - 'winner' and 'loser' might be established but it won't calm down so they won't be able to live together any more. It's not just boar-boar, it's the same for boar-sow and sow-sow but boars can just be more dramatic! It's great that they're getting used to you ☺