I think it's the same thing as always - an administrative procedural item forcing them to buy shares against their will, along with possibly some short positions getting closed.
They will attribute the run to FOMO and "WallStreetBets" pumping the so-called mEME sTOCKS in real time, but I firmly believe what we are witnessing is pent up buying pressure from the past several months being forced to be released, similar to every single quarterly options/futures rollover expiry general time period we have seen over the past year-plus (March, June, September, December).