Its dominance behaviour by the sounds of it, just asserting themselves and deciding their rank within the shoal. Should be nothing to worry about but as always keep an eye out for any injuries incurred in their bouts.
When kept in a school of at least 5 or more you will notice a few of the fish 'standing on their head'. As this type of school has a dominant leader, the Tiger Barb shows its submissiveness to others by tilting its body. You'll often see two Tiger Barbs fighting for superiority. They'll try to push each other (in a fairly harmless way) via their mouths until the looser gives up and swims away. Occasionally you'll even see three doing this.