No - he was called to the c-store before seeing them walking in the street. He stopped them because they fit the description of the people robbing the c-store.
But at this 3 PM press conference, Chief is saying this:
http://www.washingtonpost.com/news/...-and-michael-brown-did-not-relate-to-robbery/
“The initial contact between the officer and Mr. Brown was not related to the robbery,” Thomas Jackson, the police chief, said during a news conference Friday afternoon.
Rather, it stemmed from the fact that Brown and his friend were “walking down the street blocking traffic,” Jackson said.