Answer:
The leaders believe that they should get more privileges than the laborers.
Marcellus believes that the ghost of King Hamlet has been appearing, but Horatio does not believe that could be possible.
Marcellus brings Horatio out in hopes that the ghost will appear again that night so Horatio can see it with his own eyes and finally believe what they've been saying.
They imprisoned him as an act of stopping treason upon the government.