Morksmith's follow up indicates that it is the shifter that slips first, then of course the derailer follows the resulting cable movement toward a smaller cog.

So Matariki, your comment is still valid.
I think that when the cable can't slide easily within the rear-most cable-housing loop, that frame flexing tugs more forcefully on the shifter, enough to cause unwanted movement. And yes, even the cable guide down at the bb can be a culprit here by preventing the cable wire from "going with the flow" of the downtube's flexing.

But a shifter should never have to be set so stiff and tight as to prevent easy shifting(!!!).
I don't think that a bike can be ridden well in the hills like that, and I always keep my shifters oiled up, and with tension settings as light as I can get away with.
And, except for the possible scenario where a 225lb "weightlifter" is perhaps riding a 753-tubed featherweight steel frameset, I think that everybody's shifters should ideally be set to a similar, very "light" action.

So, modern chain, together with silky-smooth cabling (really inexpensive on a downtube-shifted bike), and a very light shifter tension setting should, taken together, allow most riders to shift fast and effortlessly when needed.