It's an internet rumour, so take it with a truckload of salt.
From a source that has been reliable, it appears that GOA (who look after Warhammer Online in Europe) is going to be shedding about 50% of its staff by September, while player numbers for WAR in the US set at about 97000 and for Europe (minus Russia) at about 83000. Which leaves WAR with 180000 or so subscribed accounts in its launch markets, which once contained over 800000 accounts (at launch).
WAR has been expanding, however - into Russia and Taiwan. This is likely how it appeared that WAR's player numbers didn't shift from the 300000 announced for the previous two quarters - growth into new markets hid the erosion of existing markets.
All of this fits into the latest round of free character transfers, aimed to pull characters off empty servers and onto more populated ones. And the exit of Mark Jacobs from Mythic, of course. WAR is failing badly in retaining subscribers and it probably isn't unforeseeable that EA will decide to cut its losses at some point if the game can't be made profitable.
So then, why the rumour of Mythic working on a new MMO (on top of people being syphoned off of WAR and onto DAOC)? To stop more people being fired, that's why. Start a new MMO, get the staff you want to keep working on it and then sell the hell out of it to the money men that this time they're on a winner. Move staff to the profitable MMO, just in case the unprofitable one gets cut.
But anyway: it's a rumour.