Probably because it doesn't have the CPU power to run it well in H/W mode. Might need to find a less demanding version of the video. There's info about this stuff on MX Player's FAQ:
Section #4 has the more relevant info.
Need some help, advice, something fixed? Hit me up on Hangouts @ es0tericcha0s
Now offering unlocking services - including many Sprint phone unlocks! PM for details!
HTC Droid Eris / Motorola Droid / Samsung Galaxy S Fascinate / Motorola Droid X / LG G2x / HTC Evo 3D (x2) / Samsung Epic 4G Touch (Sprint S2) / HTC Amaze / Samsung Galaxy S3 (International) / Samsung Galaxy Note 2 / Nexus 4 / HTC One / Samsung Galaxy S4 / Sony Xperia ZL / Nexus 5 / OnePlus One (64 GB)