Time domain reflectometry (TDR), pulse shape
The slower edge in the reflection indicates that high frequency signal components are in the reflection are attenuated compared to the low-frequency components.

The main reason for this is likely that the coax is lossier for the high frequency components than for the 100 kHz fundamental. A chart I found online indicates typical RG-58 has 6.6 dB/100 m loss at 30 MHz, and 16 dB/100 m loss at 100 MHz, for example. Remember that your reflected signal passes through the cable twice (so you need to consider 200 m worth of loss) when using these figures.