Transmission line: Why do longer traces cause signal reflections?
All traces cause reflections. As the trace length approaches significant fractions of a given signals electrical length, the reflection can start to interfere with the signal (constructive or destructive interference). 

You could also keep the trace length the same and increase the frequency of the signal, the effect will be the same. As mentioned in the comments, when considering the frequency of the signal it is important to note the frequency components. That is, the frequency of the fourier series components of the signal.