I understand that there were also other reasons other than the assassination, but this is widely considered the trigger for WW1. Why? And why did the whole world get involved with this one assassination and start such mass-violence?
The question seems lame for first sight, but makes sense in depth. The alliance systems weren't trivial at the start of World War 1 (or Great War).
First I recommend to check out the timeline and look for the first dozens of lines of politics in gray. The outbreak of war wasn't one instance, it was a domino effect of triggers in alliances, warnings, mobilizations, guarantees for independeces etc.
From Aplhahistory I would quote the following treaties which all had minor or major role in the participation of joining to the Great War:
The Treaty of London (1839). Though not an alliance as such, this treaty was a commitment by Europe’s great powers – including Great Britain and Prussia – to acknowledge, respect and defend the neutrality of Belgium. When German troops invaded Belgium in August 1914 they did so in defiance of this treaty, which was still in effect.
The Three Emperors League (1873). A three-way alliance between the ruling monarchs of Germany, Austria-Hungary and Russia. It was engineered and led, to a large degree, by Bismarck, as a means of securing the balance of power in Europe. Disorder in the Balkans and the Ottoman Empire undermined Russia’s commitment to the league, which proved very unstable.
The Dual Alliance (1879). A binding military alliance between Germany and Austria-Hungary, requiring both nations to support the other if it was attacked by Russia. This agreement was welcomed by nationalists in Germany, who considered that German-speaking Austria should actually be a part of greater Germany.
The Triple Alliance (1882). A complex three-way alliance between Germany, Austria-Hungary and Italy, containing anti-French undertones. Each signatory was committed to provide military support to the others, if they were attacked by two other powers – or if Germany and Italy were attacked by France. Italy was viewed as the weaker partner in this alliance
The Franco-Russian Alliance (1894). A military alliance between France and Russia that restored cordial relations between the two. This agreement also undermined the increasing power of Germany and allowed French capitalists to invest in Russian mining and industry, providing economic benefits to both nations.
The Entente Cordiale (1904). Meaning ‘friendly agreement’, this series of agreements between Britain and France ended a century of hostility between the cross-channel neighbours. It also resolved some colonial disagreements and other petty but lingering disputes. It was not a military alliance so neither nation was obliged to provide military support for the other.
The Anglo-Russian Entente (1907). An agreement between Britain and Russia which, like the Entente Cordiale, eased long-standing tensions between the two. It also resolved disagreements over colonial possessions in the Middle East and Asia. It did not involve any military commitment or support.
The Triple Entente (1907). This treaty consolidated the Entente Cordiale and the Anglo-Russian Entente into a three-way agreement, securing amicable relations between Britain, France and Russia.
For summary, the assasination itself was only a big spark in a gunpowder store which was built up by treaties, land claims, Germany's desire for more colonies, ethnic tensions and lot more.
Wow, yeah that's one heck of a question... And considering that you're talking about the foundational tragedy for a century of wars across multiple continents it's an important one to understand. The list of treaties above provides a good at the political structure that Franz Ferdinand's assassination "activated" but it is insanely hard for one work on the subject to cover the entire chain from the Old World that existed before the assassination all the way into the trenches.
But you're in luck. There's a "History Journalist", named Dan Carlin, who has done a wonderful job of this recently. His "Blueprint for Armageddon Part 1" is a journey from the streets of Sarajevo into the rational madness that follows. http://www.dancarlin.com/product/hardcore-history-50-blueprint-for-armageddon-i/
It's complex, compelling, and (currently) free.