The ship lost power in a narrow channel and ran aground. Its really that simple. Happens all the time except this time there was a bridge in the way.
Modern container ships have very little redundancy. Their only purpose is to go from point A to point B as efficiently as possible so most of them have a single low speed diesel engine that's connected directly to the propeller. No reduction gears, thus no reverse capability, e.g. the ability to slow down or stop quickly. When the tugs pull it off the pier they start the engine and the propeller starts turning. Then at some point the tugs leave and from that point forward everything relies on the ability to control the rudder to steer. Without steering the ship is at the mercy of momentum, wind and current.
