It's been allocated to allow an additional carriage to go on the Cumbria coast. The LM 153 has taken over from a Northern 153 diagram in West Yorks allowing an extra Northern 153 to be used on the Cumbria coast.
The extra carriage is permanent but the 153 is subleased. Similar to what's happened with some of the units FGW have subleased - they don't belong to them but they won't lose them without first gaining cascaded units from elsewhere.
One whole additional carriage for Cumbria - hasn't Cumbria done well? Seriously, it has, when did an extra carriage last get allocated to, for example, the Newcastle area?