A worker said they heard a "humongous crash" when the crane, which was on a tug and thought to be about 200ft (60m) high, collapsed at about 1000 GMT.
Two men were assessed at the scene but no-one needed hospital treatment.
Two other cranes toppled over at the docks in 2008 and 2009. The Health and Safety Executive is investigating.
A person who was working near the latest incident, who did not want to be named, said: "A colleague of mine said they heard a humongous crash.
"We don't know if the crane was lifting anything at the time when it crashed."
Crane driver Jay Squibb was seriously injured when he was crushed by a crane which toppled over at the docks last July.
He legs were crushed and he was trapped for more than two hours but was eventually freed and airlifted to safety.
He has since returned home but admitted he cannot see himself ever returning to the job.
In January 2008, another crane collapsed but no-one was hurt.