Why output is high even though trigger pin is high in Proteus simulation of IC555?
The trigger being low (less than 1/3 Vcc) forces the output high. But the trigger being high does not do anything to the output.

To force the output low, you must either drive the reset pin low or take the threshold pin (pin 6) above 2/3 Vcc.

Internally, the 555 is a set-reset flip-flop driven by a pair of voltage comparators.