All Heather wants to do is play fetch. Constantly. It's her normal behavior. What was new here was Mac's interest in playing ball - Mac's very intense interest in playing ball!
What was also new was Gemma's intense protectiveness towards her little brother, Mac. As soon as she understood that Mac wanted the ball, she went after Heather with a vengeance!
In her enthusiasm, she even crashed into Heather as she tried to snatch that ball from her mouth.
It didn't stop with Heather, either. Gemma and Mac were just as outraged when Hoover managed to snatch a ball!
Lexi eventually had to step in and tell Hoover to stop teasing the puppies, regardless of the fact that the puppies were five times larger than he was. It was not clear whether or not Hoover got the point.
Although he certainly knew he was being scolded from every angle, I don't think he understood that the ball was off-limits to a little squirt of a newcomer such as himself.
Once Mac had control of the ball, all he really wanted to do was flaunt his ownership of it. He taunted poor Heather mercilessly by making sure she could see it was Mac's ball now and forevermore!