The ITV region you see on Channel 103 is determined by the information on your viewing card which, in turn, is determined by your postcode. So, theoretically at least, there's no reason why it should suddenly change. (If the card wasn't being correctly recognised for some reason, your Sky box should default to showing ITV London but there's no obvious reason why it should switch to ITV Yorkshire).
It might just be that there was a problem at Granada, which forced them to show a different programme to the one which was scheduled. (e.g. they might have had a fire alarm sounding, or some other type of security alert, in the news studio, which meant that they couldn't transmit any local output and so chose to take content from a neighbouring region. I've known ITV Anglia to show ITV Yorkshire's news when such problems have occurred. On other occasions, they've shown content from ITV London instead).
Alternatively it could just be down to a temporary technical fault at Sky's HQ.
I suggest adding ITV Granada as an 'extra channel' to your Sky box. Then, if the problem occurs again, you can switch to that alternative channel to see for certain what is being transmitted from Granada's studios. (The process is a little fiddly but you'll only need to do it once):
http://www.stevelarkins.freeuk.com/itv_regions_sky_digibox.htm