Very simple. The idea was making the last normal elections an independence referendum, had independentist won. They lost.
So they decided they would fly forward. Most of them, like Mas and Puigdemont were friends or partners of Puyol, who actually manage to steal hundreds of millions of euros, probably thousands, they are going to jail anyway. They have nothing to lose and lots to win.
Puyol stole for 40 years without nobody in central Government doing anything, you could not do business there without paying to his family. He actually hired people from the Spanish secret services and got a lot of information back so central Government was scared of doing anything against him.
The plan was doing all this and after 48 hours unilaterally declare independence, because of the "Oppressive state".
For the independentists it is "now or never". Certainly is not easy to have someone as weak as Rajoy as prime minister.
For me as an Spaniard who works in center Europe who last week was in Catalunya is obvious who is using violence. It is not the police. There are people trowing stones to police, they are insulting and spitting to them, they are doing scratches to them.