Bob Stevenson (top pig vet; consultant vet to the BPA) told me heavily pregnant sows in summer often do get into a situation where they are not able to control their thermo-regulatory system and will mouth pant/dog sit and breathe from the abdomen. Apparently, the ventilation of the deep respiratory tree becomes poor and infection can gain a hold producing pneumonitis (pneumonia).
He was surprised the sow hadn't aborted sooner. Clearly a tough old bird who was hanging on!
Ah, well, at least the local hunt only charged me £50 to collect her - better than previous experiences with "proper" collection services.