TTL signal: output impedance, overshoot and other characteristics
Well your pulse starts on your 50 transmission line, flies down to the end of the cable, and slams into your high impedance scope input.  I just looked and your scope has a 1Meg Ohm input impedance.   The wave then reflects off of that impedance and begins traveling back towards your source.   When you apply the 50 Ohm terminator you have now matched the impedance of your transmission line with your cable and there is no reflection, but you will only get half of the voltage.

Take a look at this article by Johnson where he shows the reflection coefficient formula as:

 

You can see if your load impedance and transmission line impedance match there will be no reflection.

Also I think this series of three articles does a great job of explaining the details of all this.