Combos have already been confirmed to be in the game. You gain bonus xp when you kill several enemies in rapid succession, or kill several enemies with one attack. It was probably the former, which would wait for a few seconds to see if you killed any more demons before granting the XP.
As for the glitch theory, I doubt Blizzard would have released footage that included a glitch.
If you're so sure of what it is, why did you even bother starting a thread to discuss it? You expect everyone to praise you and worship you for noticing some 2 year old pointless "mechanic" that's not been confirmed as intended or confirmed as a bug? It seems like you just get your jollies by creating useless threads about things nobody cares about for the sole purpose of shooting down anyone else's opinion.
O, great Grug. Thank ye for thy wisdom that thou hath bestowed upon us! May the gods of yore reward ye with many sons and the riches of kings!