I have seen that sometimes phase plot starts at 180 deg and progresses towards 0 deg OR it starts at 0 deg and goes towards -180 deg.
Can anyone explain the reason.
The explanation of the reason for different phase functions is very simple:
In both cases the circuitry is different. (But I am not sure if you are satisfied now; but that´s the only answer one can give to the above question).
it depends which input you take as reference. As edge_tv said, when the phase starts at +180 is because you are using the inverting input as reference for the AC analysis.