Well, I just visited Lexington and it is still unclear. After the first shots were fired, many of the redcoats, in spite of Pitcairns orders, charged the minutemen, with bayonets flailing. It is true that the 70 or so minutemen who were there were mainly for show (many of them dispersed eventually, after Pitcairn ordered them to) many were shot in the back as they were retreating. However, their guns were loaded, for one Brit was wounded in the leg, and another shot in the hand.
Apperantly, the Regulars disobeyed orders because of the strong animosity between the opposing sides in the conflict, and they wanted to have a go at the "wretched, ungrateful colonials".