Damn good question, and has provided a lot of interesting answers so far. Sincerely hope it doesn't get shut down by the fun police for being too broad. ;)
I think all of the answers provided so far provide an insight into this growing trend, but there' s more to be found if we look a little closer, and dig a little deeper.Cinematic depiction of 'typical' zombie behaviour is not limited to increased speed or ferocity, but also: manner of infection, sentience, self-awareness and tactical prowess. It is nearly impossible to understand why zombies have 'gotten faster' in isolation, without considering the other behavioral changes that have naturalized themselves into the zombie genre...
It has been noted that whereas in the Romero canon all deaths result in zombiefication, modern cinema seems to privilege the curse as an infection (28 days later, WWZ, The Walking Dead). There will always before examples to counter this, but by and large they exist because they directly oppose this. Even the Romero re-set opted for this, interestingly.
Whereas the Romero canon has always alluded that zombies become smarter the longer they roam, 2005's Land of the Dead featured a semi organised, self aware and emotionally cognitive zombie force, able to manipulate ballistic weaponry. It has long been posited (and rightfully debated) that zombies themselves are an allegory for consumerism and if we choose to examine their change in behavior through this critical lens, its possible to find a possible answer..
1978's Dawn of the Dead is the most overt in said associations, featuring the undead shifting lifelessly through a shopping mall, largely oblivious to happenings around them. This film arrives from a period of relative consumer-infancy, where the post 'mad-men' boom of conscious marketing and psychoanalytic manipulation had left the masses sedated, and the radicalised minority fearful of its effects. The zombies are dopes, being pushed around by the unseen hand of their new religion: The desire to eat, to feed, to consume.
Now flash forward 35 years and look at the modern consumer: tech savvy, potently aware of the fact they are engaged in consumerism and self-schooled in navigating the nuances of this brave new world for their own benefit (Groupon, Orange Wednesdays, the list is far from exhaustive). They actively engage in their consumerism, seeking out new ways to participate and becoming willing nodes for distribution (Like!), they surrender themselves to the zombie menace and elect to run alongside the horde!
This coupled with the frenetic, ceaseless barrage of media which epitomizes consumerism, creates a thoroughly accelerated type of consumer, and thus zombie.They have evolved alongside the very entity they satirize.
P.S, I have written this entry on a tablet, which seems to have a limited allowance for hyperlinks etc. I will edit when I get home, but if someone wishes to do so in the mealtime, I would be appreciative. I'd like to imdb link to the movie titles mentioned, as well as including links to these supporting articles where appropriate... ta!