Now I see it more clearly.
In our company (737 classic) nobody turns the A pumps off for pushback.
In this case, waiting for the bar to be seen disconnected before turning the gens on, doesn't make any sense.
It's a ground personnel responsibility to remove the pin AFTER the bar is disconnected.
Should they mess up, their problem, legally speaking.
But of course, if you want to avoid troubles, you can adopt the most cautious procedure, by turning A system off before push.
In this case, yes, it makes sense to wait before selecting the generators on line, for the reason explained by Sawbones.
Now, it's our call... which of the two procedures we want to adopt?
Surely, if one operates in third world countries, the answer comes naturally.
Operating in Europe only, one might say depressurizing sys A is exaggerated... but I'm sure that if we had access to the whole record of pushback incidents/accidents, one would change his mind...
Thanks for your kind replies, Gentlemen.
LEM