This is almost certainly because you've not terminated the nport with sensible impedances. The s-parameters were probably measured assuming 50-ohm (or something similar) ports - you can see this by looking in the s-parameter file. Originally posted in cdnusers.org by adbeckett
If you connect an ac vsource directly to an nport pin, it's going to do all sorts of horrid and unrealistic things.
I just tried doing it, and I got similar results to what you describe, so I'm assuming you've just connected the circuit up inappropriately.