Chihuahuas have a ton of energy packed into their tiny bodies. You can think of it, like drinking too much caffeine. If your body is hoarding a lot of excess energy, it burns it off by shaking.
Neva loves attention. She will jump on my lap and look up at me (just like she is looking in the picture) and she will shake. I usually respond by baby-talking to her. She loves for me to acknowledge her and I always respond when she shakes, so she probably knows she can do it by shaking. I’ve read somewhere in the past, that they have evolved to shake, because people feel sorry for them and pick them up. I don’t know if it’s true or not, but it makes sense.