How do you know that?
I went to delta.com and can see that Delta 1520 is a MD-88 flying ATL-BNA and connecting to Delta 539, also a MD-88, that then goes BNA -CUN. Ground time is just 81 minutes and DL 1520 is on-time less than 60% of the time.
Without checking all possible connecting cities, how did you find it? Also, are you sure it is a direct (with a stop and number change) flight and that DL 539 is not a connecting flight (different airplane)?
Also, does DL 1520 start in ATL? If so, why is it late almost half of the time?