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Presumably it's in response to AMD's upcoming Radeon R9 290X.
The card has to be faster than the GeForce GTX 780 to have a point, but there isn't that much space between the current GeForce GTX 780 and GeForce GTX Titan. Recall that those two cards are both based on the same GK110 GPU chip, and have the same memory bandwidth. The GPU chip has 15 SMXes physically present, of which 14 are active on Titan and 12 on the GTX 780--but the latter clocks them higher.
I'd expect Nvidia to make the GTX 780 Ti into a gaming card all the way through, and cripple the GPU compute capabilities. And then I'd expect Nvidia to make the GTX 780 Ti about as fast as Titan for gaming--or possibly faster. Nvidia will likely make it a 3 GB card rather than 6 GB for Titan, and may well let it burn more power than Titan. Then the GTX 780 Ti will be priced about where it makes sense as compared to the R9 290X, and the GTX 780 will probably get a price cut.
If Nvidia goes that route, Titan will stay at $1000, but will be quite clearly not be a gaming card, since you'll be able to get the same gaming performance perhaps $300-$400 cheaper. Titan will remain Nvidia's entry-level GPU compute card.
Early pricing leaks on the Radeon R9 290X point to a price tag around $700, but that may be an overestimate, as early price leaks often overshoot the final retail price. Remember that the Radeon R9 280X is now $300, and charging $700 for a card that might be 1/3 faster than that seems excessive. It's not that AMD wants lower prices; rather, my reasoning is that if AMD were going to charge $700 for an R9 290X, they'd be charging a lot more than $300 for the Radeon R9 280X. I wouldn't be at all surprised to see a Radeon R9 290 for $400 and Radeon R9 290X for $500.