His best explanation for the big stock rally is the Fed money printing and an expectation that the easy money policy will continue.
Interestingly at PIMCO Gross hasn't seen much if any money leaving bonds for stocks.
Gross thinks that we are near the end of an epic bond market. Over the next 12 months though he sees bond prices and yields not changing much.
He isn't forecasting a bond market crash, just an end to the great bull run.