When the truck started to move the first bomber threw a grenade in order to scatter the crowd and open a path for the main bomber to get close.
The grenades didn't have the intended effect and the main bomber got as close as he could and detonated. The crowd took most of the blast, minimal damage to the truck except for one killed on top of the truck.
The truck had secretly been seriously armored and Bhutto was quite safe.
Bhutto knew that any attack would benefit her and her alone, no other way to gain that much power so early on.
The Bhutto Attacks
Cold comfort is the best we can hope for.
By Aaron Mannes
http://terrorwonk.blogspot.com/2007/10/aaron-mannes-in-nro-on-attack-on-bhutto.html