So basically the IIR filter transfer function is $H(z)=\pm H(1/z)$.
so if the poles are inside the unit circle,reciprocally poles are outside the unit circle so the system is unstable and non causal that's why linear phase is not displayd by IIR filter
The phase response is clearly non-linear, as mentioned in a comment. Note that phase jumps at the zeros of the frequency response occur depending on how the phase is defined, but these phase jumps don't make the phase non-linear. However, in the given example the phase is clearly non-linear at low frequencies.
You could also look at the corresponding ...