They've spotted a ridiculously good deal on the Canon 6D in combination with the fantastic (I know because I've got one on the desk in front of me) 24-105mm F/4.0 IS. Cinema 5D's view is that this might be a sign that another, newer full-frame camera is on the way, and you can't really argue with their reasoning.
Six years old
The current Canon full frame range consists of the 5D mk III, the 6D, and the 1DC and 1DC. Both the 5D mk II and the 6D arrived on the scene in 2012. Depending on your view of things, that's a long time or a very short time: just look at how long the 7D has been on the market. It was announced in September 2009 and was still being updated with major firmware upgrades in August 2012. This year, it will be six years old.
That's a pretty normal product cycle for cameras (although a lot are replaced sooner than that) and it's absolutely typical for other products like cars.
But cameras are now caught up in the exponential technology maelstrom. In the current environment, many people expect new models every year, if not sooner (just look at the weight of expectation that Blackmagic has on it!). On the other hand, you have the likes of ARRI, confidently releasing new cameras that are clearly eagerly awaited despite (and probably in ARRI's case, because of) older technology.
One thing that would militate against an imminent camera launch is that Japanese companies are geared away from rapid product launches. Their quality ethos is such that they have rigerous and time consuming quality assurance and test procedures that mean you simply can't rush a product out.
This special offer may be a complete anomaly rather than some unsubtle attempt to clear inventory prior to the release of a new model.
But on the other hand, NAB is just weeks away, and every major manufacturer wants to have new products to announce.
We just don't know. It's easy to forget that the market for stills cameras is many times the size of the one for video devices. With DSLRs, it will be the demands of the "traditional" users that drive the manufacturers to design and release new models.
And the both the 5D mk III and the 6D are fine cameras. They're at different price points with some distinct distance between them in terms of features and capabilities, although it's worth noting that with a good lens the picture quality is utterly similar. What I'm saying here is that there's no drastic reason to update them now, as opposed to, say, in one or two year's time.
But then, when you look at the wider picture, there is a seemingly unstoppable move towards 4K acquisition. Manufacturers without a 4K offering are looking increasingly out of line. Canon does have a 4K DSLR, the 1DC, but it's priced multiples above the 6D and the 5D mk III. We found it to be a brilliant camera, but it's not exactly affordable to anyone except the dedicated professional.
Dedicated cinema cameras
Don't forget that Canon has a range of Cinema cameras as well. These are perceived generally as
a) very good, and
b) rather expensive in comparison to the rest of the market.
You can bet that Canon spent a lot of time developing their Cinema cameras. This is not an investment that they're going to want to write off, or even overshadow, in the sort of product cannibalisation that Apple seems to happily and successfully do to their own products.
A better picture
Canon's DSLRs have always suffered from moire and softness (this is not an exclusively Canon problem!), to a greater or lesser extent. Even if you don't have any compelling reason to go for the 4 x resolution of 4K, you might want to have the choice of shooting in 4K and downscaling to HD, which will in all likelihood give you a better picture.
And it might be that Canon doesn't want to get lost in the noise from all the other manufacturers. But, they might equally say "these are primarily still cameras, and when we do upgrade them we'll do so at a major photographic show like PhotoKina".
So this is just a rumour. It will remain so until we hear anything concrete. But the very fact that the rumour could be even slightly plausible does shed a lot of light on the state of the camera industry today.