General aviation pilot reported observing a near miss on the ground while airborne with two other aircraft as a result of a runway incursion at a non-towered airport. The two other aircraft were both taking off on the same runway from opposite directions; one aircraft rejected the takeoff and cleared the runway.

Narrative

At approximately XA:00 in my Aircraft X I taxied behind Aircraft Y; SE low-wing on ramp for a departure RWY 9. Winds were 140/6. We both passed Aircraft Z at the maintenance hangar with 2-pilots engines running; just west of taxiway B as we crossed Rwy 15/33 on D to RWY 9 departure end. Aircraft Y and myself broadcast taxi intentions and radio-checked each other 5x5; plus crossing 15/33 on taxiway D intentions. A few minutes later Aircraft Z broadcast it was taxing to RWY 27.I was first to takeoff at XA:05 RWY 9 broadcasting my takeoff intentions with standard right turn south westbound. A few minutes later I heard Aircraft Y broadcast he was entering RW9 and commencing takeoff. In less than 30 seconds Aircraft Z advised he was entering RWY27 for takeoff. airborne; I broadcast a radio check directly using Aircraft Z (in case his receive volume was turned down?) and he replied 5x5. I then broadcast that there was Aircraft Y on a takeoff roll RWY9. Aircraft Z advised 'Well I am in position RWY27'.Immediately Aircraft Y broadcast he was aborting his takeoff roll and vacating RWY9 I believe he said at Taxiway B (1/2 way down the runway).Aircraft Z I believe created a runway incursion and other aircraft took evasive action with an Aborted takeoff. The pilot made no further apology or comment.