The left atrial pressure tracing here is not the best part of this figure. It might have some pedagogical value if it were discussed in the text, but it's not.
Measuring left atrial pressure directly requires a puncture or a pathological condition (e.g., a septal defect). This is why you'll typically see pulmonary capillary wedge pressure instead of direct left atrial measurements. This tracing corresponds more to a simplified rendering of what would be expected in a wedge pressure tracing, vs. direct left atrial measurements. The fluctuation appears to be an exaggeration of the response to pulmonic valve closure. As discussed in the same chapter in Costanzo, on inspiration, the pulmonic valve closes after the aortic valve. This would correspond to splitting in the second heart sound and, at times, a brief blip in wedge pressure. It is not nearly so clear in an actual tracing, though.
If you look a little lower in the figure, you'll see the fluctuation you're interested in corresponds with the tail end of the second heart sound.