I would guess it is to do with the PTU... as the PTU senses the rise in the new operating engine hydraulic system pressure it will disengage, and at the point of disengagement there MAY not be full hydraulic system pressure available for brakes for a brief perid of time. I think, anyway, just an educated guess... we certainly don't apply any brakes on shutting one engine down taxying in; in that case if the PTU didn't kick in then brakes would be on the secondary system.
Don't understand the SOP of taxying out on one engine, though, but mine is not to reason why....
Squid