The mirror actually reflects front-to-back. Because this is a property unique to mirrors, and not otherwise observed in the physical world, our eyes/brain interpret this effect as a rotation. Since humans' eyes are more often used as a left-right pair (when sitting or standing) rather than an up-down pair (when lying down), as witnessed by the fact that the preferred direction of reading text is predominantly horizontal, this rotation is almost always viewed by the brain as about a vertical axis rather than as about a horizontal axis. However, a reflection is not a rotation, but is equivalent to a rotation combined with either a left/right (normally) or top/bottom (unusually) swap, so the mirror image appears swapped, or "back-to-front".