I have created a differential channel with Transmission line and interconnects for simulation. In the real channel, there is 100nF capacitor in between 2 transmission lines. Now the problem is when I put this capacitors in series while connecting the transmission line (TL), I see very steep transition of S21 as well as S11 at low frequency (around kHz level). but If I remove the capacitors the transition is not there at those very low frequencies. I am not sure why is this transition happening at low frequencies while capacitors are placed? I have given the pictures below:
Your transmission line has an impedance of (possibly) 50 ohm and it is terminated in 50 ohm hence the two capacitors (net value 50 nF because they are in series) form a high pass filter with the 50 ohm load of the t-line.
The 3 dB point of 50 nF and 50 ohm is about 63.7 kHz and pretty much occurs where the slope is in your first graph.