"Fortunately, we were able to come up with technology that makes it much harder to game the system."
Possible answers - a sudden increase in number of AU tradelines brings up a flag, time since tradelines were added could be considered, time between adding tradelines. Both should be easily incorporated, as could a "delay" before a new AU tradeline could be incorporated into the score.
I am sure there are other ways. But piggybacking should be fairly obvious - no one spends 5 years getting ready to piggyback!
The slide from grace is really more like gliding
And I've found the trick is not to stop the sliding
But to find a graceful way of staying slid