It is doing two things wrong, it is moving a constant value to the stack and it's doing it before increasing the stack pointer, so it's momentarily outside the

It is doing two things wrong, it is moving a constant value to the stack and it's doing it before increasing the stack pointer, so it's momentarily outside the stack area. If an interrupt hits during the instructions the stack is broken, the interrupt service routine will increase the stack for its own use and overwrite that value.