Effect of large capacitor for differential channel in S-parameter simulation
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.