There are (naturally) conspiracy theories.
The official line is that a letter to a member of Parliament tipped them off and that Guy Fawkes was caught in the nick of time by guards.
But, there is a conspiracy that says that the guards knew about the plot well in advance and made a point of apprehending Guy JUST before he went through with the plot, thus rallying the public behind the King.
Remember, remember, the 5th of November, gunpowder treason and plot...