Maybe instead of asking "How come?" she would have liked you to ask, "T, your next client isn't here yet and usually you give me extra time if the next patient is late. You aren't doing that today and I'm wondering what the reason for that is?"

Maybe if she knows what is behind the "why" she would be more apt to give you a reason?

I don't know though, I'm not a mind reader of t's unfortunately. I wish I was.