Why does accelerating electron emits photons? I have read that accelerating or oscillating electron emits photons. But why and how does it so? And why only photons? There are other bosons like gluons, W and Z bosons, so why does electron emit only photons? And what is the mechanism ?
 A: From a classical point of view, if you look at the field lines created by charge at rest or at constant velocity you'll see straignt lines. Now if the charge changes its speed (i.e accelerating), a "ripple" will appear and propagate along those field lines. Check the animation on this page :http://www.tapir.caltech.edu/~teviet/Waves/empulse.html. You'll notice that the lines remains straight "before" and "after" the little acceleration that the charge made, but in-between there's a variation that is percieved as an electromagnetic wave, and so a photon in a quantum point of view
A: This might not be quite the answer you are looking for, but one useful way to think about it is: the accelerating electron emits photons because nothing forbids it from doing so.
By definition, because an electron has electric charge it is coupled to the electromagnetic field, and is able to produce excitations in this field which we can call photons. This is literally what it means to have electric charge, so there is no need for a 'mechanism' beyond that. The electron also has weak force charge (isospin), but no strong force charge, so right away we can tell that it cannot directly emit gluons simply because it is not coupled to this field (By the way, don't ask me why an electron has some kinds of charge and not others).
From this, the possible emission channels for an electron are those of the electromagnetic and weak forces: photons, and W and Z bosons. However, W bosons can be ruled out immediately: they have electric charge, so if the electron gave them off it would violate conservation of electric charge. But photons and Z bosons are both neutral, so those are okay.
What else is forbidden? Well, an electron that is at rest cannot give anything off, because that would be creating energy. But the principle of Lorentz invariance tells us that if an electron at rest cannot emit anything, then it also cannot emit anything when it is moving at a constant velocity, because one can always choose a reference frame moving at a different (constant) velocity without changing an observable like whether something was emitted.
Nothing so far has forbidden an electron that is accelerating from emitting a photon or a Z boson, so we should expect that both of these can happen. However, because a Z boson is quite heavy, to emit these without violating energy conservation the electron must be accelerating to a degree that is only possible in a particle accelerator (or similar natural process). Photons can be as low energy as you like since they are massless, so at the energy scales we are used to only emission of photons is possible.
This process of deciding if something is possible by elimination might seem odd, but it's actually an extremely useful perspective in particle physics. Indeed, it has its own Wikipedia page, and an associated saying popularized by Nobel laureate Murray Gell-Mann: "Everything not forbidden is compulsory."
A: The why is due to the principle of conservation of energy. When the field that is responsible for the acceleration is electric--the gravitational force is several orders of magnitude weaker than the electric force, and electrons are not nucleons--work is done by an electric field, whose quanta is your favorite boson--you guessed it--the photon!
The mechanism is called Brehmsstrahlung, which loosely translates to "braking radiation" in German. It is defined as the process of an electron undergoing an acceleration--negative values corresponding to decceleration--which causes a change in energy. The emmited photon, due to conservation of energy, has the energy as calculated from work.
The Wikipedia page has a very detailed and simple explanation of the process: Bremsstrahlung.
A: To get accelerated, any body as well as any particle needs energy to be applied to this body or particle. This happens through photons or more common through electromagnetic waves. This is obvious in particle accelerators and it happens in every acceleration too.
Throwing a ball with your hand you do not touch the ball in the meaning, that the molecules of your hand touch the balls molecules. The outer electrons of both bodies prevent this and this electrons intermediate through photons. At the end some energy is over given to the accelerated body.
During the time, the moving body is not influenced by any force, it moves straight and does not loose more energy as it get from the  surrounding world. This it called thermal radiation and it happens anytime.
When one try the slow down or to move a particle in circle or when ever the particle loose energy, when happens what you asked, it radiates photons. This is the inverse process to acceleration.
Gluons and W and Z bosons are intermediate processes inside the nucleons and are not observable in our dayly live.
