A piece from Talking Points Memo (link) gives reason to think that an ill-considered pre-emptive police action helped provoke the violence and destruction seen in Baltimore. Quoting:
... The police assumed that black youth in local gangs were targeting them and that some sort of violent purge was imminent, so they began painting a picture of imminent threat. Convinced that they were under attack and having sufficiently defined the enemy (i.e. black youth) to the press, the police decided to strike first.
Therefore, police deployed cops in riot gear to Mondawmin Mall to cut off the buses that the children from local schools use to take home before the children got out of school. From there, things descended into violence as frustrated children, trapped on city streets by armored police and cut off from their mode of transportation home, began hurling rocks and bricks at the police.
The article isn't particularly objective, but if what it says is true, it points to yet another example of bad decision-making on the part of authorities. A get-tough approach may appeal to one's instincts, but it doesn't always work as well in real life as opposed to the movies.
To be clear, I'm not suggesting that this justifies the rioting on the part of some protestors. For more on that, see this previous post.