There is two things I might believed could happened:
First, when you said you closed first reg, did you actually screw it all the way in until it bottomed, and left it there? If you did, you might have stopped the air entering the plenum, and when you took a shot, there was not enough pressure to hold the valve back. And when the air slowly leaked in, the valve was already open. You must be wery carefull when screwing the reg all the way in, and stop the moment you feel it has reach the bottom, as you might damage it.
It is probably a good advise to open the reg slightly, before pressurising system, to let the reg piston have room to move. No matter what, a reg is never supposed to be closed.
The seccond thing wich might have happened, is that you had to much hammer spring force from your previous tune, and forgot to lower it when you had reduced pressure. If you reduced reg pressure "allot", but did not reduce hammer spring force also, the force may be able to knock the valve pin out of position.