Why is ZnFe2O4 a normal spinel?
The reason stated in the literature is fairly straightforward. However, actually tracking down the original source is harder than it looks. 

Going back to Phys. Rev. B22, 115 (1980) one finds in the opening paragraph the statement 


  Mixed Zn ferrites of the type Zn$_{x}$M$_{3-x}$O$_{4}$ where M
  is a magnetic ion have been the subject of numerous
  investigations in the past, especially with regard to
  their magnetic properties.$^{1}$ The metallic ions occupy
  the tetrahedral A or octahedral B sites in the spinel
  crystal structure. The Zn ions preferentially occupy
  the A sites because of their tendency to form covalent
  bonds involving sp' orbitals.


Reference number 1, on the first sentence, is a book [J. Smit and H.P.J. Wihn, Ferrites (Wiley, New York, 1959)]. The statement on $sp3$ bonding is not backed up by a reference. I can only assume that it was considered common knowledge in the spinel community in 1980.