Gathered here are the remnants of the Jedi Order. The Jedi Masters have already sacrificed themselves, and most of the remaining Jedi have died on various battlefields. So far, only thirteen Jedi Knights remain in the Jedi Order, and they are the ones who first suspected Palpatine.
As for the reasons these Jedi Knights are suspicious of Palpatine, it's quite simple. Many of them have experienced the previous war between the Galactic Republic and the Independent Federation, and thus discovered that Palpatine seems to always send them to battlefields with no hope of survival. Although outwardly, Palpatine claims that only the strength of the Jedi can turn the tide, not all Jedi Knights believe his nonsense.
They have long suspected that everything, including the fall of the Jedi Council, is part of Palpatine's conspiracy, but they simply have no evidence!
