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IIRC, this has been discussed in the past on this forum.
The problem is that when a pax asks what triggered the alarm, the TSO has no way of knowing if the pax is an innocent person trying to educate himself on how to improve the checkpoint process for everyone or a bad guy 'testing' the system.
If hypothetical 'bad guy' has three questionable items, TSA doesn't want to reveal to him which one(s) got caught by the xray. That just makes it easier for 'bad guy' to find a way to avoid detection the next time.