My theory on why MK is so crowded while the other parks are less so is because Universal is drawing vacationers away from WDW.
Let's face it, you're vacationing in Orlando and decide to spend a couple of days at Universal on what used to be an all-WDW vacation. Which WDW park are you least likely to skip?
I imagine Universal is putting the hurt on DAK but especially DHS. Although TOT is one of Orlando's best attractions, DHS has pretty much always come up short when compared to Universal Studios. Now that WDW vacationers are giving Universal a try due to the popularity of WWOHP at IOA, they're more comfortable with giving US a try as well. And, of course, US has had some really good additions over the last few years with the best yet to come.