Explanation for hyenas asymmetric legs length
First most animals have limbs of different lengths. consider a house cat at full extension its hind limbs are much longer than the forelimbs. 

The relative length of the limbs means almost nothing functionally, it is the relative length of the gait, or the distance the limbs can swing that is important, a longer less flexible limb may swing the same distance as a shorter but more flexible limb. Source

When the limbs have very different gaits you end up with hoppers or so called pendulum runners like rabbits and kangaroo or they do not fully utilize the possible arc of one limb like gorilla this second one pops up when ground travel is not the primary means of locomotion. Hyena do fall in either category their limbs are close enough for a normal gait. 

Hyena limbs are not as different in length as many people think, the massive neck and shoulder muscles make the front limbs look taller than they are in many photographs, but yes they are longer. The heavy musculature plus the longer forelimbs are all adaptation to make the animal stronger with more leverage, better able to rip apart a carcass and crush bones. They do not need the massive hind limbs of the large cats becasue they do not "pounce", you see the same adaptation in the so called "Fanged Cats" or saber tooth cats they have massive forelimbs. More importantly the massive muscles on the shoulder and neck make the forelimbs less flexible so to get the same arc swing out of the limb, it has to be longer.