Humps under the insole

This is an odd one but the right shoe of each of my three last/current pairs of trail shoes have developed a strange hump under the insole. In two shoes the hump is under the ridge of my toes, in the other it's at the very tip, effectively making the shoe shorter and causing bruising and blisters on my longest toe. In over 30 years of running I've never come across anything like it. On the first pair I just assumed it was a defect in the shoe, but now it's happening to the third pair, that seems less likely. Has anyone come across anything similar? Any suggestions of how I may flatten out the humps? More importantly, any ways of stopping it happening in future? While the first pair had already clocked up plenty of miles and I didn't mind retiring them a bit early, the others are only halfway through their expected mileage.