Is your tap water high in iron? If it is, it would cause yellow to brown rust stains in your sinks and toilets. So if that’s the case, then maybe the water left in the bristles after rinsing your toothbrush is leaving little rust deposits at the base of the bristles. OR - do you rinse your toothbrush thoroughly after brushing?
Too much sugar in one's diet is usually the cause of a toothbrush turning yellow. Sugar tends to exacerbate plaque buildup in your mouth, resulting in a more yellowish appearance on your teeth. If you wash your teeth frequently, this deposit may transfer to your toothbrush, turning it yellow.