This behavior usually is due to some asymmetry in a differential circuit. In your case, this asymmetry probably comes from the post-layout extraction. I have also seen this behavior during mismatch Monte Carlo analysis.
The attached pdf file shows a simple circuit that duplicates this behavior. It models an amplifier with some coupling from the output to the positive input. Depending on the amount of coupling, you get either 180 degrees or 0 degrees at low frequencies.
Like ssahl already mentioned, this behavior usually does not indicate a stability problem. To convince yourself (or others), you can use the Nyquist stability criterion (see
http://www.designers-guide.org/Forum/YaBB.pl?num=1244840188 and
http://www.designers-guide.org/Forum/YaBB.pl?num=1217822985).
In my example circuit, the strange behavior is due to the fact that there is a second loop in the circuit that does not include the stb probe Vprobe. If I redraw the circuit so that Vprobe is part of both loops (see the second circuit in the attached pdf file), the strange behavior disappears.