I would guess too much low protein food - this from the Grim, UTI thread , link provided , but applies to this situation "Australian veterinarian Ian Billinghurst, whose book Give Your Dog a Bone introduced the BARF (Bones and Raw Food or Biologically Appropriate Raw Food) diet to dog lovers around the world, describes how to adapt his menus for urate-forming dogs in a report posted at several websites.
“In Western countries today,” he says, “I am led to believe that a typical homemade diet for stone formers would contain about 80 percent rice, 10 percent vegetables, and 5 percent meat. This is an appalling diet to feed any dog. This is borne out by dogs forced to endure it. They suffer from numerous problems including continual hunger, a lack of energy, poor coat condition, and difficulty in maintaining weight or severe losses of weight.” Such a diet is not only deficient in protein, fat, vitamins, and minerals, he says, but it does not prevent stone formation.
The raw meaty bones Dr. Billinghurst recommends are chicken necks, chicken backs, chicken wings, and turkey necks. “Use plenty of puréed or pulped vegetables,” he says, “including lots of leafy greens. The diet could also include eggs, cottage or ricotta cheese, yogurt, and olive or flaxseed oil, supplemented with vitamin B complex, vitamin E, kelp, and a teaspoon of cod liver oil several times a week.” Cod liver oil is important for urate-forming dogs fed a homemade diet that does not include liver.
Feeding a changing variety of eggs, cheese, dairy products, and small amounts of medium-purine meat, poultry, and fish along with low-purine vegetables, fruits, and supplements – as well as ample water to keep urine diluted – can help any urate-forming dog stay healthy and happy."
the diet you are mentioning for your dog is difficient - especially if the rice is white , and hasn't been soaked or sprouted to remove phytic acid which inhibits mineral absorption.