Around Pad 39B there was plenty of evidence of ice formation. The entire tower was covered with icicles. Challenger, the SRBs and External Tank had been on the Pad for a total of 38 days. Within that there had been 7 inches of rain. There was a large chance that water had gone into the joints of the SRBs and damage some of the O-rings.
Putty performance is another possible cause of the joint seal failure.
Putty, Zinc chromate is placed on the inside of the joints before assembly. It is there to stop the heat of combustion gas from going to the O-rings. Putty is also forced between the gap of the tang and clevis, to make sure that the seal is tight. The hot gases can make holes in the putty, thus letting gas go through to the O-rings which could cause damage. Second, the putty moves by gas pressure and could go all the way to the O-rings.