If you were riding in the rain, you were riding in the mud. All that water coming off the street had particulate matter in it -- dirt, grit, petroleum products, fertilizer, whatever. All dissolved into a nice suspension just perfect for wearing out your pads.
Get Kool Stop salmon pads. They resist this sort of thing a lot better than most OEM pads, particularly Shimanos.
RichC
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Training: 2002 Fuji Roubaix Pro (105 triple)
Commuting/Daytripping: 2001 Airborne Carpe Diem (Ultegra/XTR, touring wheels)
Commuting/Touring: 2000 Novara Randonee (Sora/Tiagra/LX, fenders, lights)