Heat could have caught up with you. And yes, when one failed, the others would have been hit with that much more heat resulting in quicker failure of the others.
This is all particularly true if you were running off the speed controlled convection output of the Coal-trol. By running the fans at lower speeds you may well have accelerated their failure in that hot environment.
When you get them replaced I'd suggest increasing the CFM setting to its maximum (50) and if possible moving the fans further from the stove. Also check to see what your fans are rated at for ambient heat and make sure they are well above the max of what you're going to encounter at the position in the pipe you mount them. No way to tell what this is going to be without running them, so I'd suggest getting one fan, mount it and hook it up and run the stove full out for a while then go to the CFM screen, this will run the fan at the minimum speed it will run which should create a worst case heat environment for a given fan, measure the temp in the pipe before the fan and see if it is in spec for the fan. If it is over, increase that CFM setting to its maximum value and take a reading again to see if it improves or not.