Royal Gramma Aggression

Hi everyone. I have a 30g fish only setup with a common clown, Royal gramma and two pyjama cardinals at the moment. It's been running for a couple of months with no issues but recently I've had some concerning behaviour with my gramma. He has a particular cave which he protects and is fine but recently he's been all over the tank bullying the cardinals by charging at them forcing them to stay in a tiny top corner of the tank. They come out for feeding but I'm concerned that the gramma is now seriously stressing them out. The clown and the gramma seem to be best of friends, always swimming with each other. Why is he suddenly picking on the cardinals and what can I do to help them? :(

Thanks