Sometimes when a cat grooms another cat, it’s nice. A cat helping another cat out, cleaning those hard-to-get spots on the back of the neck with a friendly lick.
Other times? It’s more like, “Hey, I want that nice nap spot, and I’m going to annoy you until you give it to me.”
I have a cat that grooms passive-aggressively. If she doesn’t think the other cats are up to her very specific grooming standards, she’ll groom them until they are, (whether they like it or not).