NIST specifies that implementations were required to create Monte Carlo test vectors for ECB and CBC mode for the AES competition.
What's the reason for these tests? The only thing I can think of is that it is some randomized testing of the block cipher in specific modes. However, in that case the difference between ECB and CBC would not matter.
It seems to me that this is a weird way to test for implementation errors. You'd expect things like bounds checking and such, but just performing the encryption over and over again may not actually hit the right results. And weird keys / input values are also covered by the other tests.
Finally, having the algorithm generate it's own input seems dangerous. If the block cipher doesn't behave as specified, then the input may be erroneous. So why use the block cipher as DRBG and not another well specified / tested algorithm?