The real reason behind this seems to be that the Janjaweed have stepped up their raids in Chad, so this is no longer an "internal matter" and the Sudanese government cannot avoid being seen as the bad guys.

BBC news: Chad proposes anti-Sudan alliance (17 November 2006)

Chad has offered to send troops to the Central African Republic to help fight rebels it claims are backed by Sudan.

Since a rebellion began in Sudan's Darfur region nearly four years ago, violence has spilled over its borders into eastern Chad and northern CAR.

Prime Minister Pascal Yoadimnadji said as Sudan was attempting to destabilise both Chad and CAR there should be a "general mobilisation" of the people.

Those whom the Gods wish to destroy They first make mad. -- Euripides