Why did my hen start being violent?

I have 2 female silkies who are both 11 months old. They are inseparable but this morning when I let them out of the coop, the larger hen started attacking the smaller one. She started pecking at her and wouldn't let her get up until I put her away in the run.

After 5 minutes or so I let her out and they went back to being close together but the little one was still scared. She wasn't cut or anything but she was always very nervous and now she is even more scared.

They have been together their whole lives. Do you know what could have caused this behaviour and how can I prevent this from happening again? I've only had them for about 2-3 months and I'm only a beginner with chickens.

Thank you!