Seems the police wanted disturbance. Maybe they thought it would take the heat off them.
Meghann Harris, a teacher at a nearby school, described on Facebook what happened:
Police were forcing busses to stop and unload all their passengers. Then, [Frederick Douglass High School] students, in huge herds, were trying to leave on various busses but couldn't catch any because they were all shut down. No kids were yet around except about 20, who looked like they were waiting for police to do something. The cops, on the other hand, were in full riot gear, marching toward any small social clique of students…It looked as if there were hundreds of cops.
The kids were "standing around in groups of 3-4," Harris said in a Facebook message to Mother Jones. "They weren't doing anything. No rock throwing, nothing…The cops started marching toward groups of kids who were just milling about."
A teacher at Douglass High School, who asked not to be identified, tells a similar story: "When school was winding down, many students were leaving early with their parents or of their own accord." Those who didn't depart early, she says, were stranded. Many of the students still at school at that point, she notes, wanted to get out of the area and avoid any Purge-like violence. Some were requesting rides home from teachers. But by now, it was difficult to leave the neighborhood. "I rode with another teacher home," this teacher recalls, "and we had to route our travel around the police in riot gear blocking the road… The majority of my students thought what was going to happen was stupid or were frightened at the idea. Very few seemed to want to participate in 'the purge.'"
A parent who picked up his children from a nearby elementary school, says via Twitter, "The kids stood across from the police and looked like they were asking them 'why can't we get on the buses' but the police were just gazing…Majority of those kids aren't from around that neighborhood. They NEED those buses and trains in order to get home." He continued: "If they would've let them children go home, yesterday wouldn't have even turned out like that."
Meg Gibson, another Baltimore teacher, described a similar scene to Gawker: "The riot police were already at the bus stop on the other side of the mall, turning buses that transport the students away, not allowing students to board. They were waiting for the kids.…Those kids were set up, they were treated like criminals before the first brick was thrown." With police unloading busses, and with the nearby metro station shut down, there were few ways for students to clear out.
Several eyewitnesses in the area that afternoon say that police seemed to arrive at Mondawmin anticipating mobs and violence—prior to any looting. At 3:01 p.m., the Baltimore Police Department posted on its Facebook page: "There is a group of juveniles in the area of Mondawmin Mall. Expect traffic delays in the area." But many of the kids, according to eyewitnesses, were stuck there because of police actions.
http://www.motherjones.com/politics/2015/04/how-baltimore-riots-began-mondawmin-purge