I always consider Nico Nico my home away from home. The amount of stuff like this is almost hard to comprehend when you go there.
The amount of series thrown in here is too funny though. After I heard them making the “usoda” joke, I figured a Higurashi scene was coming. Didn’t see them doing what they did though. AKA strange facial expressions mixed with Rena’s character song.
They’re just mixing up everyone’s voices. Kugimiya Rie (Nagi) did Shana, hence the Shana reference, Nakahara Mai (Tia) did Rena, hence the Higurashi “USO-DA”. And Hayate is well, Hayate, for obvious reasons.
This video requires at least moderate levels of moe-anime contamination to understand, so I don’t blame a lot of people for not getting the references.