Female schnauzer rubbing her butt up and down on my male schnauzer. Both are fixed.

I have a 14 year old male schnauzer who is fixed. I wanted to get him a playmate, so I just adopted another schnazuer this evening. They get along perfectly. She is fixed as well. She is 18 months old. She keeps whining and rubbing her butt up and down on my male schnauzer. It's weird. My male just looks at her and walks away. And she follows him and does it again and wines. I asked her ex mom if she really was fixed. She said yes, over a year ago. Why is she doing that? Is it a submissive thing?

Both are schnauzer/shitzu