Boots not made for walking!

When Bb was packing to go to Dublin I suggested he take his walking boots and travel in them - practical and comfortable. He agreed but as he was walking round the house before he left I noticed he was leaving behind black bits on the floor. I thought it was soil from the soles, but thought it strange as they hadn't been worn for a while and had been stored on a top shelf in our wardrobes. He then noticed that they were totally falling apart, with the soles and uppers separating! After he left I examined mine and they looked ok, but when I put them on, they started to do exactly the same! Those in the blip are mine. We have had them for a while (many years) and haven't used them for years, but I have no idea why this would happen. Too hot in the summer months perhaps? Anyway, both pairs are now completely unusable and irrepairable.