thats becuase you are thinking that the scr is being triggered by current rather than voltage. you can transmit an electrical charge without having to complete a circuit with wires (think radio waves etc.). basically, the scr gate has a very high impedance. this means that its very easy to induce a charge onto this wire and raise its voltage. with a floating wire attached, it acts like an antenna. any stray em can induce a voltage and bam. scr turns on. when you touch it with your hands, static electricity. bam. scr turns on. when you look at it funny, bam, scr turns on. the pulldown resistor makes sure that no charge is induced without a small amount of current flowing (which should only happen when you purposely induce a charge), and when that current stops the gate is discharged through ground. this is more or less whats happening.