Why the pitch rate q is zero while angle of attack is changing and vice versa?
Why the pitch rate q is zero while angle of attack is changing and vice versa?


It generally does not (*). You misunderstood the statement in the Etkin's book.

The image you posted come from the chapter about "the $q$ derivatives" $C_{L_q}$, $C_{M_q}$ and $C_{h_q}$.

The book is quite explicit:


  These derivatives represent the aerodynamic effects that accompany rotation of the airplane about a spanwise axis through the C.G. while $\alpha$ remains zero


It then presents


  Figure (b) shows the general case in which the flight path is arbitrary


(this is the figure you posted)


  This should be contrasted with the situation illustrated in figure (a), where $q=0$ while $\alpha$ is changing.




The book in no way implies that either one case or the other is true, it is only presenting contrasting academic examples to illustrate the concept of what the $q$ derivatives are.



(*): I said generally because the book itself present one case where you CAN have constant $q$ and $\alpha = 0$, the steady pull-up.
Again, this in no way implies that you always have 


  q is zero while angle of attack is changing and vice versa