I just made an account to post my experience with this issue:
I have a rmbp 13 2014 model and it happened in all el cap versions. Everytime i remember it happening there was youtube involved and alot of times clossing the tab.
I too am using a 13" mid-2014 rMBP and have been having the "freezing" issue since updating to 10.11.4. My usage of this machine is strictly for work. That said, unlike others, I am not watching YouTube videos so for me, the "freezing" cannot be tied to this. I will say that the "freezing" did occur twice back-to-back for me this week (within an hour), and in both situations the only applications open on the computer were Mail, Word 2016, ChemDraw Mac, and Acrobat DC Pro.
I have not had this problem occur on my nMP, but run a different set of applications there (again, no YouTube videos).
I am hoping for a solution to this problem soon, otherwise I will restore 10.11.3 from a backup.