It's because the birds keep eating the rice and don't know when to stop. The rice expands in their stomach and causes them to explode when they eat a lot of it.
Sorry anonymousey, it is UNTRUE that birds explode after eating rice (rice plus moisture is the same volume after they combine - check your levels the next time you cook rice). A better theory is that the birds that came to feed, left a mess and tried to take shelter in the churches, along with brides and guests possibly taking a dive after slipping. If you still don't believe it is untrue, please check the news next June, during rice harvests to see if they caught the pesky fellows exploded like popcorn all over the fields on camera.
LOL! I think birds are getting confused with horses, who will have terrible colic after eating pure oats or alfalfa. It isn't good to feed birds bread, though, because bread mold spores get caught in their throats and cause serious problems.
I'm not a bird expert, but I did look up the info on a bird website, that basically said that is a myth. Another website suggested it came from a newspaper general advice columnist that it hurts birds. No clue on horsies either way.
Didn't we stop using rice at weddings in the states because birds tended to explode? All I could think of seeing him feed a bird rice, is "DON'T EXPLODE!".