Here's a video clip of the small container vessel MV Mentari Crystal capsizing at the dockside at the Port of Tanjung Perak in Indonesia last Monday. Note the explosive snapping of a mooring line 9 seconds into the video. It wouldn't have been healthy to be caught in the whiplash!
She ended up with her hull (probably her bilge keel) resting on the bottom of the harbor. Getting her out of there is going to be interesting, what with the threat of pollution from her fuel tanks.
I've no idea what caused the capsize. I doubt it was overloading alone, although that may have been a factor. If she sprung a leak, it'll be even more difficult to shift her.