One of the earlier actions in the Volcanic Ash QRH drill is to CLOSE the thrust levers, thus reducing the turbine temperatures, to satisfy lomapaseo's concerns. This action would also tend to reduce both the temperature and pressure of the bleed air, so a Bleed Trip Off is very unlikely.
Like Barkingmad, I believe the idea is to increase stall margins within the compressor, rather like the 'bleed-band' system used by some gas turbine engines.