Why are action potentials propagated in only one direction down an axon?
Biologically, action potentials are propagated in one direction due to how neurons are connected to each other. Signals are transmitted across synapses to eventually the soma of a neuron. These potentials are summated in either positive or negative ways and transmitted to the axon hillock. The hillock is where the signal begins to propagate down the axon. There is no comparable system of signal transmission in the opposite direction.
Experimentally, however, we see something completely different! If you apply a depolarizing potential to the axon hillock, then the signal will propagate in the correct direction. However, if you apply that same potential to the middle of the axon, the signal propagates in both directions! If you think about it, this is exactly what you would expect, considering how the ion channels work.
Any channels in the vicinity of the depolarizing potential will open causing depolarization, thus propagating the signal. The only way that channels in the vicinity of a depolarizing potential are not activated is during their refractory period, which explains why pulses travel in one direction instead of reflecting in any significant way or causing a single shift to one voltage!
I hope that helps!