This is the movie canon and they have decided Cool is the way to go. When irrestible force meets immovable object, other stuff gets blown away. In the Mighty Marvel fashion, Nuff Said! [TVtropes: The Rule of Cool.]
For the canon seekers:
The Marvel Cinematic Universe, designated Earth-19999 is not the same as the Marvel Comics world designated Earth-616 where the canon stories for Marvel characters are said to take place. The MCU has made changes (or established their own canon) which contradict or expand the expectations of characters in that universe.
In the MCU, Captain America's shield is not composed of Primary Adamantium (a mixture of vibranium and adamantium, making it the most indestructible thing on Marvel Earth).
The MCU shield is instead composed entirely of Vibranium (or a vibranium and other metallic alloy added for strength).
In the canon works Vibranium absorbs and negates vibrations or applications of kinetic energy such as bullets, explosive force or god-wielded hammers. It has not, however, as shown in the movies, exhibited an ability to deflect, reflect or otherwise redirect energies spent on it. In the canon works vibranium negates and dispels such energies without any display or reaction at all.
NOTE: This is a physical misnomer since bullets deflected by a device which can absorb ALL kinetic energy should drop harmlessly to the ground in front of the shield. Instead, they are deflected away from the good Captain both in the comics and the MCU. A case of having cake and eating it, I would imagine. Perhaps the Rule of Cool trope is being observed. (Obligatory TVTrope warning displayed.)
In the comics, Cap's shield and Thor's hammer Mjolnir have come into physical contact on numerous occasions, without the reverberation effect seen in the Avengers movie. So we are left to assume this display of energy redirection is a side effect of shield meeting hammer and dispersing the energy a bit more energetically and omnidirectionally along the edge of the shield. This comic entry shows Thor striking Captain America's shield, really hard.
- In the MCU, it appears the Captain's shield will if struck hard enough, redirect the energy that strikes it, outward toward the rim of the shield. And now we know. Admit it. It did look cool. Note the gong sound...
There is one other thing we could blame this strange deviation from canon.
- What if the shield did exactly what it was supposed to do and deflect Thor's blow?
- What if the splash effect is the excess magical or lightning energies being displaced and redirected away from the shield?
Then no one is "wrong".
- The shield blocks and protects Captain America
- the vibranium keeps him from turning into a pile of broken sticks
- the shield just displaces the blast wave of lightning away from everyone.
Hurray! No canon has been extremely violated.
Though we then get to ask, what do they paint that shield with...? Not a scratch on it.