I'm guessing from the success of re-release of The Lion King in 3D last month, Disney and Pixar had a thought "hey we could totally take more money from families by re-releasing our other amazing classics in 3D."
2012
Beauty and the Beast (January 13),
Finding Nemo (September 14)
2013
Monsters Inc. (January 18)
The Little Mermaid (September 13).
press release after the jump