I have never heard of the term rosing, but maybe others have. It could also be a silly word the family member made up. It is possible it was playful behavior by someone who had a really bad sense of boundaries or a skewed sense of what is appropriate touch with a child. I had cousins that would jump around the corner and say "boo" to make me jump. I can see how someone would pinch a child to make them jump in a playful way - but grabbing the butt? It's inappropriate to do it with the butt. It kind of reminds me of how guys in a locker room might do stupid stuff like slapping each other's butt with a towel. And yet, it's different, because it's an adult, doing it with children.
It's unwanted contact with a part of the body some would call a "private part." There is the idea that if a swimsuit covers it up, no adult should be touching it in an unwanted manner - unless there is a true medical need, or needing to change a diaper, or something like that.
I wouldn't do it to a child, and if anyone did it to my child, I would tell them to knock it off, pronto.
That all being said, I would be careful about reading into it too much. It could easily be a playful and inappropriate behavior at the same time.
Regardless if it's playful or not, if it came up from a body work session, it may be affecting you now, and that's the most important thing: how you felt about it back then and now.