Slowing the bike down with the front brake once leaned over causes the front suspension to compress which changes the handling geometry and puts more load on the front tire which then wants to push through the corner instead of maintaining an arc.
Several years ago a guy fell down in front of me on an exit ramp because he touched his front brake while on the ramp. He just got some scratches and bruises. His is speed was no more than 20-30 mph.