It should only be happening when they aren't showing someting in HD on the simulcast channels like C4HD
'HDMI wide' will only stretch an SD channel because there is no picture outside the square.
HD always broadcasts in widescreen. If they then show a square program like Friends they broadcast the black borders down the left and right sides to make the broadcast widescreen but keep the program the right size.
See here for more info
http://en.wikipedia.org/wiki/Pillarbox
It shouldn't happen on BBC HD as they only show HD widescreen programs