CNS last page i posted a huff po link with a police presser. in it the Chief gave a brief, poorly organized timeline. The cop *could* have heard the suspect description right before the fatal confrontation occurred.
Also as I noted the most favorable accounts to the officer point towards the j-walking stop. Some brief pause or break in contact, possibly coinciding with the radio call, and the resulting series of altercations which led to the shooting and death of Mr. Brown.