Unfortunately, neither article gives enough detail to say why this happened. But according to several commenter eyewitnesses on the Herald blog, the bicyclist got a wheel caught in the trolley tracks in the street.
Were the bicyclist and bus eastbound or westbound? The #39 bus westbound makes the left turn from Huntington Avenue onto South Huntington Avenue — see MBTA map of the route (click on “Interactive Street Map” tab and zoom in). A westbound bicyclist would be ill-advised to overtake a bus here. The bus driver should be signaling the turn.
I got an e-mail passing along information from the Boston Channel that the bicyclist was trying to pry his wheels out of the tracks and was struck by the rear of an eastbound bus. Eastbound is not what I expected, and a bicyclist wouldn’t be struck by the rear of a bus unless it was backing up. More likely, the side of the bus.
What lanes were they in?
Did the bus scrape the end of the bicyclist’s handlebar? If he was going faster than the bus, that would have turned the handlebar toward the bus and dumped the bicyclist to the left; or vice versa.
How close was the bicyclist to the side of the bus?
Was there other traffic besides the bicyclist to the left of the bus?
Did the bus merge toward the bicyclist?
Was the bicyclist going faster than the bus, or vice versa?
What kind of investigation is underway?
Many bicyclists unreasonably fear the traffic from behind and ride close to the side of a bus, leaving them entirely defenseless if the bus merges toward them. When I was new to urban riding, that is what I did — or sometimes, I would pass a stopped bus on the right — risking collisions with passengers getting on and off the bus. I knew this wasn’t safe, but I didn’t understand yet that overtaking collisions are rare, and the safe thing was to claim the space I needed for my safety.
My fundamental rule about passing large buses and trucks: merge out before reaching the bus, and stay at least 5 feet away from the side of one — claim the entire next lane if needed — so you are where the bus driver can see you and expects traffic, and so you can brake and fall back if the bus picks up speed and/or starts to merge toward you. This is the safe way to pass a bus — and it is often necessary to pass one, because buses frequently stop.
Trolley tracks need to be crossed at a sharp angle, or jumped over. It’s best to avoid them where any other complication might occur.