[J.A.Sucker]

The magnets stay parallel to each other "in sync" because they want to be aligned as such (think about the poles in a magnet - the North of one magnet wants to be near the South of the other and vice versa). This is going to always be true for the pair, so if you spin one, the other will track it.

The second half of the question is about torque and gravity. Before you spin the magnets, they are at equilibrium - there's enough friction between the magnets' coatings and the glass that the attraction between the magnets lets them stick together on the glass. If the glass or coating of the magnet was more slick, then they may just drift down spontaneously.

Once you start spinning them, the magnet pair always moves down once you disturb this equilibrium due to gravity and due to the torque that you apply to the system, combined with the magnetic force field lines that are radiating from the magnet pair. If you could spin them fast enough, you could impart enough torque to pull them upward first, but that may be tough here becasue the magnetic fields are present. This is actually the trickiest thing to think about, but basically the magnetic fields extend out away from the magnets and they serve as a guide for the magnet pair. If you spin it clockwise, you tap into the one that would pull the magnets "right" and if you spin it counterclockwise, you tap into the field line that pulls it "left." Again, since the system starts in a delicate equilbrium, you can tap into either path depending on which way you spin them. The reason that the magnets slip and move down is because frictional forces are always harder to overcome at the beginning, but are easier to maintain movement on - kind of like watching me get off my ass to get a beer on a Sunday afternoon. Once I get off the couch, I can get to the fridge quickly, but that initial momentum is the hardest. Same thing here - once you spin the magnets, there's enough momentum for gravity to overcome the frictional force.

There ya go.

Dr Sucker