Q:

Why are the G20 summit protests in Hamburg so violent?

Compared to other G20 summits in the past the protests in Hamburg, Germany seem quite violent to me. There were similar occassions of violent protests, such as at the 27th G8 (2001) at Genua, at the 33rd G8 (2007) at Heiligendamm, Germany and at the 4th G20 (2010) in Toronto. But they are years apart, so protests of this extend are not a standard occasion.
I understand (as is stated here) that most of the protests concern global warming and capitalism. However, there are ominous statements from protesters like this one:

We are living in a democracy and the red zone is not a democracy. I'm sure we can't stop all these leaders from meeting but if we can stop them from getting their food or catering shortly we've achieved something.

Now that protests are as natural to such events as the diplomacy done there is quite clear. 
But the violence (there are reports of Molotow cocktails, tear gas and so on) strikes me as uncommonly high given the circumstance that firstly, the summit takes place in Germany and secondly that this single (annual) meeting most probably will not change the course of the world forever.
My insights are mere, could therefore someone elaborate on the reasons that spark those rowdy protests?
(I feel like the answer to this question does not apply to Hamburg in particular, for example the quoted statement above was given by German student without obvious immigration background.)
I am asking primarily about actions of protesters.

A:

The location of this year's summit played a huge part in the ability to stage protests. The summit is held in the centre of the populous city of Hamburg, so protests are easier to be carried out, as compared to past G20 summits. One such example would be China, which gave residents a week-long holiday and encouraged them to leave the city during the G20 summit in Hangzhou, which "guarantee(d) a trouble-free summit".
News articles from Reuters and NPR have suggested that Merkel had hoped to show the world that protests tolerated in a healthy democracy.

Since then, G20 summits have been largely peaceful. This may be because some locations -- such as Brisbane, Australia -- have been a long way to travel, or because protesters have been kept far from the summit venue amid tight security. Last year, China ramped up security for the summit in Hangzhou and reportedly gave residents a week's vacation.
Source: http://edition.cnn.com/2017/07/07/europe/g20-hamburg-protests/index.html

The protesters are mainly anti-capitalists and organised mainly by an organised group called "Welcome to Hell". They hope to disrupt and cause inconvenience to those involved in the summit.

Christian, one of the protest organizers who also declined to give his surname, said the "point is to disturb the G20," adding: "It's not right that a few countries get to decide what happens to the rest of the world at this summit."
Sunday's demonstration was organized by a group called "Protest Wave G20". Other demonstrations this week are called "Welcome to Hell" and "G20 Not Welcome".

There are many small groups protesters that advocates from many different platforms, ranging from fighting climate change to addressing worldwide economic disparities.

As the leaders began holding informal meetings, thousands of protesters from around Europe, who say the G20 has failed to solve many of the issues threatening world peace, poured into Hamburg to join the main demonstration.

The city also has a radical left-wing scene who organise protests occasionally.

Many streets in and around the city center — which is famous for its extreme left-wing scene — were blocked to ordinary traffic, though nearby public transport stations were open, albeit with increased security patrols.

And the protestors are also said to be associated with the black bloc which had protested in many parts of the world and recently participated in the May Day protests in Berlin. May Day protests had occasionally turned violent in Germany.

