Some people in this game seem to think it amusing to jump into someone's battle wait for their enemy to add and then flee leaving the original battler with not 2 but 3 enemies to face ( sometimes 4 as I've seen a pair of them doing it together. It seems trivial but could actually be considered bullying and there is no easy way to report this as if they say nothing you can't pull up their profile to report and even is they do say something you don't have room to explain what happened. It is really annoying to have to run back to where you were and fight the battle again because someone thought they were being funny. This would be especially harassing to those on the lower end of the level zone who sometimes have a problem with just 2 opponents.
