Time domain reflectometry (TDR), initial pulse slope
The slope on a shoulder (first plateau) is defined by interplay of resistive and dielectric losses in the transmission line (cable), see the iconic source, by Howard Johnson. In your case the resistive loss effect seems to dominate.

There are also 13,000 hits on Google for [tdr lossy transmission line], from Tektronix and Agilent and other good places.