In 2012 a group of researchers collected a large amount of RSA moduli and calculated their greatest common divisor in order to find common factors between them. By finding a common factor they could the divide and completely factorize the key. They concluded that the vulnerable moduli where generated by PRNGs with inadequate randomness. Does it necessarily mean that those key are not random?
The prime numbers for those keys were generated from random number sources with bad entropy. Entropy is really just a human measurement of how unpredictable something is. Entropy sources responsible for the common RSA factors hadn't had enough "random stuff" input to them to get mixed around, so they output data that tended to be the same across many devices. Some more info