I don't know if we have to go reaching for lead as the reason.
If the effects of chemical substances on the brain are indeed a factor... and yes, they probably are. Then there is probably a far more simple explanation.
Drugs.
Instead of police being faced with impoverished young men of the 70s, 80s and 90s who are hyped up on cocaine, crack or pcp and trolling for their next hit. They are dealing with the impoverished young men of today, high on pot and trolling for snacks. That is, they would be trolling for snacks if the munchies are ever able to override their desire to keep playing whatever video game.
If we consider the issue in a rational fashion for a moment, it's not terribly surprising that Rodney King was hyped up on PCP and trolling for another hit, whereas Trayvon Martin was high on pot and trolling for snacks.
And, again, when one tries to consider the matter rationally, these two people represent two fundamentally different elements. With predictable impact on statistics when their stories are "writ large" in a society.
If your crime is coming from demographic groups like this... then your crime will probably change with the changing drug use pattern.
It's not a popular notion, because it smacks of "evil"... but drugs really can be used to determine and direct the course of a society.