Hundreds of protesters stormed the Swedish embassy in central Baghdad in the early hours of Thursday morning, scaling its walls and setting it on fire in protest against the expected burning of a Koran in Sweden.
Even the most peaceful person is going to get a little pissed if you burn something they hold sacred, mockingly, and then get ready to do it several times again. (After you've seen the distress it causes)
It's not the damage to the book necessarily, but the complete lack of respect and intention to cause anger.
It would kind of be like burning a Constitution and American Flag on Independence Day in front of the Capitol or maybe Washington Monument. Or lighting a cross on fire in front of the Lincoln memorial on Juneteenth or MLK Jr. Day. Or setting up Swastikas in front of synagogues during Passover.
It's essentially intentionally spreading hate. Now, if you burn a Torah, Qu'ran, and Bible as an anti-religion demonstration on some random day of the year, that's a completely different thing. But doing such a demonstration on a religion's holiest of days in front of that religion's places of worship is a very different matter.