Basically, go watch every Rick Hartley video on YouTube. He is awesome at explaining. It will become intuitive instead of being a bunch of cargo cult advice. (Normally I dislike long videos but this is absolutely worth your time if you design PCBs.)
ADD: in short, it happens for "any" frequency. This is how a voltage signal propagates. However, for DC it happens once because you see "change" in voltage only once and that's it. As you increase the frequency, you'll end up seeing more voltage waves due to "change" in voltage. That's when "reflections" come into picture - happens due to discontinuity in impedance.
https://youtu.be/QG0Apol-oj0