Take this as rumor, although at least in my opinion it seems pretty logical, and is therefore more than likely what happened.
via StraightSilver
Basically an email was sent around to GW managers the day the new Dark Eldar releases were put up on pre-order (the Talos, Venom etc).
That email was meant for managers only and had a pic of the Razorwing Jet Fighter in which wasn't meant for public viewing at that time.
When GW Nuneaton posted pics of that weeks new releases on their store Face Book page the manager inadvertently (or not?) also posted the pics of the Razorwing.
They were on the internet within minutes, which then meant that most people were talking about the Razorwing rather than the other new shiny stuff that they were supposed to be talking about.
Admittedly this pic was only leaked two weeks early as the Razorwing is out 19th June, but i suspect it made the higher ups at GW question whether or not managers should get these little teasers.
Trouble is it was these little perks that always used to make being a GW manager that little bit more exciting, but I guess now they are encouraging their stores to have their own web presence they can't afford to let mistakes like this happen too often.
Of course this is my speculation, but it comes as too much of a coincidence timing wise for me to think it's not related. Not that I want to put the blame at Nuneaton's door specifically, I saw that email too and I haven't worked for GW for over 15 years!