Just like a coin, there is an obverse die and a reverse die made by a die engraver to strike the silver centers used in silver strikes.
During the striking process of the silver centers, sometimes one (or both) of the two dies move around in the loose die holders (rotate) ... which is what causes the misaligned silver centers. Then the pre-struck silver centers are pressed into the brass outter ring on silver strikes. The degrees of rotation can vary widely which is why each individual rotation is not considered a separate variation (in my opinion).