A common tactic during the late 1500s and early 1600s for soldiers with slow-loading firearms was for each rank to step forward, deliver a volley, then retreat to the back of the formation in order to reload.
If reloading a musket needed to be done while stationary, and it presumably took far longer to reload than it took for the next rank to step forward, fire, and retreat behind the first rank, how would a formation 10 ranks deep maintain a constant rate of fire without drifting backwards?
A reply to /u/hborrgg
This question asks about one of the most unfortunate aspects of that time period: there was very little primary material available that discusses such details. On one hand, there was a brisk business in selling "military manuals", but those tend to focus only at high level tactics (how to move squadrons, how to set the formation of battalions), or at the lowest level (such as de Gheyn's instructions in the use of pikes and firearms). But among these "military manuals" it was not known which exact ones are used in each unit.
At the practical level, how to set formations such that they do not drift in the wrong direction, was the job of the squadron's officers: lieutenants and sergeants. They instructed the troops how to rotate, whether to fire in volleys, how to change directions, how to move as a unit. There was tendency in the Spanish army to keep squadrons and companies together so that they would train in such matters and develop the necessary processes, drills, and language. Thus, the exact means of how to adjust, whither ranks moved forwards or backwards or what-not, are determined by the NCOs through squad-level drills and practices.
I shoot a matchlock musket and I disagree with the premise that they could only be reloaded while standing still.
Edward Davies in The Art of War and England's Traynings describes five different ways for a 10 rank deep formation of musketeers to give fire.
Firing while winning ground: https://gyazo.com/c65756900c5e498765c522d84574d953
Firing while losing ground: https://gyazo.com/37793042b0a5642e54e0353ee8ca9c9e
Firing to the left or right while advancing: https://gyazo.com/41b6faa3e61df0bd2c80f9eecb9aed50