What is the cause of propagation for an EM wave? I am having problem to imagine this in a graphic way. 
When I think of some mechanical wave, for example, the sound, I can see why it propagates: the first layer of air pushes to the second one, etc. So when point X1 oscillates, it pushes or drags point X2 next to it, and hence transmits the oscillation to the next point. 
But with an electromagnetic wave, what is causing the transmission between point X1 and X2? I guess that, when the E changes, it generates a B field, but this is in the same point. I can't see what is causing transmission from X1 to X2.
 A: First, you need to understand that an EM wave is a disturbance in the existing EM field. 
Please see this answer here:
https://physics.stackexchange.com/a/107396/132371
Now you have to understand that you are looking at this the wrong way. You are asking whether a photon is traveling in space, and how.
But the photon is traveling in spacetime, not in space. In spacetime the photon is not traveling, its worldline is lightlike. That means, that the photon is not moving in the time dimension. 
It is moving in space, but not in time.
The spacetime interval is zero, because the worldline of the photon is lightlike. It is not traveling in spacetime, the emission and absorption is happening directly.
It is us, who have rest mass who can observe the photon traveling in space with speed c.
But in fact it is us, because we have rest mass, who are traveling compared to the photon in spacetime. We are moving in the time dimension, that is why we can experience time. 
A photon by us is observed to be traveling at speed c. But in fact it is not traveling in spacetime.
Please see here:
https://physics.stackexchange.com/a/189257/132371
Now you are asking how it can propagate in space, but in fact there is no propagation in spacetime, only in space, and that only tells us that there is a EM wave that shows that there spacetime reduced to zero to transfer momentum.
It is because we are always moving in time, but photons are not traveling in the time dimension, for us it looks like the photon was traveling in space, but we are moving in space relative to c. It is just that we see the photon traveling in space because we are moving faster in the time dimension, but that just shows us a lightlike worldline in space.
