Nothing but an educated(ish) guess. Panasonic are obviously unable to meet demand for their own branded screens. If the contract is due to end in February then (and I repeat this is conjecture rather than known fact) either Panasonic want all the panels to themselves, or Toshiba need more panels that Pana can supply and so have gone elsewhere.
Sorry I can't be more definite than that but all we've actually been told is that Toshiba will be branding Panasonic panels until February and thereafter it will be NEC.
I believe Toshiba have signed a long term deal with NEC and also Sony to jointly invest in new plant and technology - cant remember all the details; its available in a press release on the NEC (Japan) web site.
As far as I can tell the next Toshiba will be based on the NEC MP3 - but February is a long way away in Corporate world these days so all may change in the interim.