Nope, it was early in game development and it looks to me like 1 character animation has to finish before the next one can start. The WD had some buff or debuff hit him just before the level-up. Once that animation finished, the level up animation that had been queued was able to start.
Yeah it was a little delayed, but I really doubt that we're going to see that with the release.
If they do some kind of "combo bonus" with exp gains I doubt it's going to be a cumulative bonus that's applied at the end of the fight but rather an exponential bonus that's added to each individual kill.