I cannot speak for the 500ft, but I think that the 8000ft comes from two seperate issues:
Sometimes Mode C/S, particularly in dodgy areas, cannot interpret the readouts from A/c climing or descending that quickly (may also apply to the 500ft, as the arrow indication may not appear, thus making it difficult for safety / coordination / mil ops)... and potentially just removing the C/S completely.
The second reason I have been told may apply to W2D and fast EGNO climbers setting off TCAS that often that enough reports were filed to make some sort of response.... This is completely anecdotal but does make a lot of sense (I think?)