Along with seat belt backup, that tower also 'tilts back' prior to (or just entering) brake run so high G's are pushing body straight down into seat. It really surprised me Icon has their tower hit brakes still tilted forward, which puts extreme amount of force on the exact portion of OTSR's which allowed guest to slide out. Prior to accident I assumed it also tilted back during drop, but nope.
*of course we'll only know reasoning after full investigation. Video and picture clearly shows how high OTSR's were prior to ride, and moment he slid out was when braking started.