Mercury retrograde is the most widely recognized astrological term in American usage. The astronomical event it names is well understood and entirely apparent rather than physical.
The motion is an effect of overtaking
Planets orbit the sun in the same direction, and inner planets travel faster along shorter paths than outer ones.
As one planet overtakes another, the overtaken body appears from the moving observer to slow, reverse, and then resume its usual direction against the fixed stars.
The same effect is familiar from passing a slower car on a highway, where the other vehicle briefly appears to move backward against distant scenery.
Mercury does it often because it is fast
Mercury completes its orbit in a fraction of an Earth year, so the alignment producing apparent reversal recurs several times annually.
Each retrograde period lasts a few weeks, and periods of apparent slowing before and after are described by astrologers as shadow periods.
All planets except the sun and moon show retrograde motion from Earth, and outer planets spend substantial portions of each year in it.
The astronomical puzzle it once created
Retrograde motion was a serious problem for models placing Earth at the center, since planets had no reason to reverse in a system of simple circles.
Ancient astronomers addressed it with epicycles, small circles carried along larger ones, which reproduced the observed paths with considerable accuracy.
Placing the sun at the center resolved the loops as a consequence of relative motion, and retrograde became a viewing effect rather than a behavior.
The astrological meaning was assigned separately
In astrological tradition, a retrograde planet's significations are described as turned inward, delayed or requiring revision rather than as reversed.
Mercury's traditional associations with messages, commerce, travel and reasoning are what produced the modern popular expectations about communication and technology.
Those associations come from the tradition's symbolic system, not from the geometry, and astrologers themselves describe the two as distinct layers.
Why the term escaped astrology entirely
The phrase has become common shorthand in American speech for a period when things go wrong, used by many people who follow no astrology at all.
Its usefulness is partly linguistic, supplying a compact, blameless explanation for a bad week that requires no accusation of anyone.
Practicing astrologers frequently note that the popular usage bears little resemblance to how the condition is treated in technical practice.
The phrase also spread because its predictions are unfalsifiable in casual use. Minor mishaps occur in every three-week period, so the label always finds something to attach itself to.