The issue of pressurizing hyds at the gate has to do with the possibility/probability of squashing someone who happens along and sticks some part of himself in harm's way as you pressurize.
If you are operating a narrowbody that sits close to the ground and has "stuff" drooping, you'd better be careful ( 727 Krueger flaps...any open gear doors, drooping TE flaps on any type acft ).
We had mechs killed in these situations when727 LEDs snapped closed after they had "inserted" themselves into the leading edges for some purpose and someone else turned on a "B" pump and opened the gorund inteconnect for yet another purpose without coordinating with all concerned. At least it wasn't a crewmember who did the deed but a fellow mech.
Unless you fly some big, tall airplane, you better be darned careful about pressurizing hyds at the gate without letting EVERYONE know what you're about to do.
The most dangerous time to operate an airplane is from the time the main gear touches the ground until the main gear leaves the ground again.