Why do increasing concentrations of zinc chloride not linearly decrease the freezing point?
You've reached the other side of the eutectic.

See, that graph of lowering freezing temperature is always a part of a bigger picture.



Once you are past that minimum, it is no longer freezing point that you are measuring. Instead, it is the point where the solution becomes saturated and $\ce{ZnCl2*6H2O}$ (or whatever hydrate is stable at those conditions) starts crystallizing.

You'd see a similar pattern with pretty much any soluble salt. Zinc chloride, if anything, is good in that it dissolves immensely well, so the graph is wide, and its features can be seen more clearly.