There are a number of possibilities that may cause dry, pithy fruit on your tangerine tree. I do not know your irrigation schedule, but with our rather dry years that we have had, you may need some supplemental water. During the summer, established citrus trees should be watered deeply (to a depth of 3 feet) every 10-14 days. The problem could also be the late harvest of the fruit, or the rootstock on which your tangerine was grafted (tangerines are especially susceptible). This may not happen every year. Cold weather, or exceptionally hot fall weather can also be a culprit. Experts recommend that good cultivation practices can help reduce the problem. This would include fertilization, early harvest and irrigation.