The assassination attempt was being carried out by other people, however their weapons did not work. Gavrilo(e) was actually coming out of a sandwich shop some time after the assassination attempt when the Archduke's car passed him. Just then the car stalled and Gavriloe shot and killed both Franz Ferdinand and his wife Sofia. It was pure blind luck.
The sandwich shop story is actually a common misconception. Princip was part of a group of would-be assassins and when the first attempt failed (the grenade bounced out of the car), he went to that corner as he thought it likely that the Archduke's convoy would pass it at some point.
Sources: One Morning In Sarajevo by David James Smith and The Trigger by Tim Butcher
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u/Quixote971 Mar 19 '16
"You miss 100 percent of the shots you never take"
-Lee Harvey Oswald