I like the rust and moisture absorbtion explanation, particturlarly if all 4 wheels are locking. If the car was put away with the brakes adjusted snug, the lining will adsorb water over the winter. Especially in a garage shared with a driver car. After being driven in the summer the linings will be dry from being heated periodically. They will adsorb water in a damp garage over the winter and swell some. Maybe just backing off the adjustment slightly will solve the problem. They could then be readjusted after being driven for a while.
Loren Nelson 300G