Why longer stubs cause more intense ringing?
Facts:


The signal isn't sinewave, it covers a wide frequency range. 
A stub works as a resonator. Multiple reflections ring a long time at frequencies which have wavelength 4x the length of the stub or that divided by an integer (NOTE the wavelength must be calculated by using the signal velocity in the cable.) 


So, let the lowest resonant frequency of a stub be Fo. The longer is the stub the lower is Fo. Also 2Fo, 3Fo, 4Fo.... are proper resonant frequencies. The longer the stub the more resonant frequencies fit into the frequency range of the signal, the higher is the total resonant reactive power.