Why is $y(t)=x(t/2)$ a non-causal system?
Because for negative values of $t$ you have, for example, $y(-2) = x(-1)$ which depends on a future value of $x(t)$ at $t=-1$ for the current value of $y(t)$ at $t=-2$. 

Note that $t=-1$ represents a future time for $t=-2$.