Is this a problem of late? Perhaps she is in heat and the feeding is what gets her stimulated, much like grooming some geldings can stimulate them to become erect.
If you were to give her some hay at a different time than usual, in addtion to her normal feeding, would she still urinate? Would she if she were to get grain? I think it's possibly just a habit of hers. Many horses that are stalled prefer to urinate in their stalls and will hold it until they come in after turnout. One theory is the splash factor(soft bedding and hay absorbs the urine and keeps it from splashing on the legs) and perhaps she was stalled when younger and associates being back in the bedded stall with hay feedings.
You could always have the vet check her, but if she's not showing any other signs of illness or metabolic issues, it's likely just a quirk. Personally, I would probably try little experiments like feeding her some hay again later in the day, or pull her out of pasture and feed her by herself, or if she got a carrot or something to that affect.
Other signs of trouble, metabolicly speaking, would be if she's obese, signs of Cushings, such as a persistent shaggy coat, no matter what season it is, if the urine is exra thick, off colored or she seems to strain. Or if she's prone to founder, and thrush problems.
Otherwise, I wouldn't worry too much about it at this point.