This is because of wing anti-ice.
If you get a problem with one bleed due to bleed trip off or eng. failure, you have to switch off the pack on the affected side.
The wing anti-ice situation though is a bit contradicting. In the bleed trip off c.l. is written 'avoid icing conditions', in the eng. fail./shutdown c.l. not. May be because N-1 you are altitude limited (weather).