According to one interview I saw with a Baltimore Police Officer who covered that Beat :
Mr. Gray was in an area that is a regular active hotbed for drug sales . The bicycle Cop saw Freddie conduct a " hand-off " with another person also known to the Police .
They believed that Freddie had just made a drug -sale so they chased him down . They believe that as he took off from the Cops that he dumped his stash of drugs and cash .
They referred to Freddie as a known drug dealer and a " frequent-flyer ". Also he was known to almost always " put on a show " as he was being pinched and often he got " Jailitis " meaning that he claimed to need medical attention in order to delay / avoid going directly to the slammer .
Could be that he cried " Wolf " once too often and was not being taken seriously .