I assume that you are talking about what happens in Chapter 11.  This is the place where Jim Hawkins is in the apple barrel and happens to overhear the pirates talking about their mutiny.

For the most part, the pirates do not give any specific reason for being upset.  Generally, they just do not like being disciplined.  Israel Hands, for example, does not like the captain "hazing" him.  The rest of them just want to be in charge and to go off hunting for treasure.  Silver mocks them for this, saying that they are too headstrong and don't think enough about the effects their actions might have.  He wants them to wait until the time is right.