Don't usually pay too much attention to the media hype but:
"One passenger on the plane said the aircraft shuddered violently before going into a brief nosedive."
Sounds to me (rather frighteningly) like the onset of heavy buffet followed by a stall recovery?
Don't fly the 146 and don't want to speculate, but for what other reason would the crew push the nose forward so violently?
This is the
BBC News Online story.