Warning! Long windy post ahead: *dusts off soapbox*
Personally, I think officials are grasping at straws. They delayed the recall and announcing the contamination as long as possible so they could determine where the problem was before they announced it, and now that the recall happened people are demanding answers and officials are scrambling.
I find the melamine "added for increasing protein" story ridiculous. Protein is simply amino acids. Some stories online claim that melamine can "trick" a NIR(near infra-red) spectrometer into reading the nitrogen as protein. I find that hard to believe as well. Spectrometers are extremely accurate. In addition, melamine would be an expensive alternative to cheap protein sources such as chicken feathers.
I have two personal theories that I've been waiting to see any other chemists or officials mention.
IF melamine is the true culprit.
ONE: Melamine is known to migrate to food when heated. This is why plates and cups made of melamine cannot be used in a microwave. Here is an abstract from a study done in Britain several years ago which demonstrated just how strongly this occurs:
...migration of melamine and formaldehyde, monomers used in the production of melamine-ware food contact articles, has been determined from 50 retail articles purchased in the UK. The food simulant 3% aqueous acetic acid was used as this is the most aggressive simulant towards melamine plastics. The test conditions used were repeated exposure to the simulant for 2 hours at 70°C, since the articles were all intended for general use including contact with hot foods and beverages. Melamine migrated from 43 of the 50 samples tested and formaldehyde migrated from all 50 samples.
http://cat.inist.fr/?aModele=afficheN&cpsidt=16897306
In addition, melamine, when heated causes
the loss of nitrogen, leaving behind guanidine, a pharmaceutical product used as a muscle stimulant and ammonia which is toxic.
It may be possible that cargo containers or holding facilities used for wheat or rice were made of melamine and became overheated.
In addition, there is a German chemical company called Degussa that makes a cement product called Melment. The brand name MELMENT® combines melamin resin, glues and adhesives, and cement to form a concrete product. Again: a storage facility may be leaching melamine.
TWO: Melamine and cyanamide are related. Cyanamide is used in everything from fertilizer to pesticide. Both of which could be sources of contamination if used on crops.

*steps off soapbox and joins the
grasping
for
straws
club* otherwise known as
G.F.S.C -- Good For-nothing Scientists Club