WHAT CAUSES IT?

[Esteban]

Is not friction.

[Qiaozhi]

Is not friction.
Which one are you asking about?

Let's discuss the copper pipe, as it's really straightforward to explain.
As the magnet falls down the pipe, due to the action of gravity, it induces eddy currents in the copper wall of the pipe. This current, in turn, generates it's own magnet field that opposes the field of the magnet, this causing it to slow down. The same thing is going on in the other non-ferrous metal pipes.
As for the plastic pipe - that should be self-explanatory.
I don't understand your comment about the superconductor. What happens there?
I'm guessing that, due to the Meissner effect, the magnet never falls to the bottom. Is that what you're saying?

[Esteban]

Which one are you asking about?

Let's discuss the copper pipe, as it's really straightforward to explain.
As the magnet falls down the pipe, due to the action of gravity, it induces eddy currents in the copper wall of the pipe. This current, in turn, generates it's own magnet field that opposes the field of the magnet, this causing it to slow down. The same thing is going on in the other non-ferrous metal pipes.
As for the plastic pipe - that should be self-explanatory.
I don't understand your comment about the superconductor. What happens there?
I'm guessing that, due to the Meissner effect, the magnet never falls to the bottom. Is that what you're saying?

If pipe is made by superconductor magnets never falls.

[Fred]

It is easy to "visulize" if you think of the pipe as an infinity of small loops of wire (thus in short circuit) : just as with a motor or generator, if you short circuit the terminals it gets heavier to turn.

[Qiaozhi]

If pipe is made by superconductor magnets never falls.
In that case, it's the explanation I gave earlier. The Meissner effect expels the magnetic field from the super-cooled pipe, and the magnet is effectively left floating.