A huge spike in PPS would jolt him and force him to close his short position, like when a frog is placed in a pot of boiling water and jumps out immediately.
However, SIRI is on a steady but SLOW incline.
As the price rises in small increments, he doesn't sense his losses so he keeps shorting. He stays in the water that is slowly boiling. He can't even detect the imminent destruction of his bank account so he gets fried.