Odd Behavior. Suggestions?

[Bailey_]

As some of you know I'm currently working with an inter-dog aggressive female black lab.

I regularly switch up the dogs I bring to her class (only ever well-minded, extremley obedient dogs) and one in particular is another black lab named Cooper.

At this point in her training, Andi knows Cooper very well. But tonight during their lesson I noticed an incident that I have seen once or twice before over the course of the past couple months.

She will react when Cooper pee's, or goes number two. By 'react', I mean if she is in a sit/stay and notices that he's peeing, she'll get up and lunge towards him. No barking/growling.

Sometimes she'll just do a little head lunge towards him, and until redirected will give him the 'stare down'.

I have a few theories as to her behavior, but I want to make sure I'm not on the wrong track. Thoughts?