I've been wondering the same thing.
The story of the guy rummaging in his pack on the bus did not feel right. Why rummage through a pack that's about to go off.
And where's all the "Allah is great" (not!) shouting before the explosion?
If the bombs were set with timers, there was absolutely no need to be suicide bombers. And if they were suicide bombers with wristwatches, there was no need for timers.