Because they kept firing rockets. 2008
During the initial week of the ceasefire, Islamic Jihad militants fired rockets on Israel. Under pressure from Hamas, Islamic Jihad had agreed to abide by the temporary truce, which was meant to apply only to Gaza, but had balked at the idea of not responding to Israeli military actions in the West Bank. The New York Times reported that the Islamic Jihad action broke the Hamas-Israeli Gaza truce.[3] During the next 5 months of the ceasefire, Gazan attacks decreased significantly for a total of 19 rocket and 18 mortar shell launchings,[3][4] compared to 1199 rockets and 1072 mortar shells in 2008 up to 19 June, a reduction of 98%.[5]
Pretty sure. A similar thing happened in 2013 too.
The Geneva convention specifies what sort of things are valid military targets. Posters aren't military equipment, ammunition is. Hamas can put as many fliers, books and posters as they want in hospitals and it's not a problem. It's the Ammo depots that make it a military target.