I don't think the root is shareholders. These numbers are getting absurdly high, and so it's predictably being driven by commercial banks and debt. And lo and behold, the US monetary supply is skyrocketing out of control - up ~6% in a year, more than a trillion dollars, driven primarily by commercial lending. [1] As the bubble starts to shake all we need is for them to start packing the debt in asset backed packages and we're good to go, or at least as good to go as 2008 was.

[1] - https://fred.stlouisfed.org/series/M2SL