Firm making me do 2 more interviews after giving verbal offer?

Hey guys, was hoping to get your thoughts on this, as I'm a little puzzled.

A week ago, after going through 6 interviews, I was given a verbal offer over the phone by the director of a megafund PE firm. They've since been "preparing the paperwork for me to look at". However, a couple days ago HR just asked to schedule two more interviews. HR says they are "informal interviews for more team members to get to know you". The interviews are with junior/mid-level people (Senior Associates). I'm interviewing for a more junior Associate role. 

I just had one of the above interviews, and it was actually tough - completely threw me off guard. They asked me detailed questions on technicals and my deal experience.  

I'm wondering - what could be going on here? Have they changed their mind and they need more datapoints? I've learned my lesson and treating the next interview seriously. But does anyone have any experience with this type of behavior? Does this happen? Can anyone shed some light?