Melissa Peterman who plays Barbara Jean was pregnant in real life with a child (born in Oct. 2005) when these episodes were filmed. That is how they explained why Barbara Jean was getting fat without showing her character getting pregnant. When the show went on hiatus in early 2006, Melissa lost the weight.
The other reason why they went one more season is that WB signed a contract with Reba's producers through the 2006/2007 season. That contract was signed 2 to 3 years ago before CW was even formed. If the CW wanted to cancel the series (which they tried), they would have paid big fines for breaking the contract. They decided to air the ordered 13 contracted episodes once some of CW's other new shows tanked on Sunday's at 7 pm. The contract also has a provision for the network to order additional episodes if the ratings were good. I have not heard if that has been done. In any case, CW is getting their money's worth by airing the new episodes and the previous years episodes to recoup the cost of the original contract.
I personally like the show and hope it continues. It is already running in syndication on the "Lifetime" network and some local affiliates.