I am not the expert on splitting citrus fruit, but, it could be environmental factors, such as rain, humidity, uneven temperature swings and watering. Fruit swell under those certain conditions, and judging by the picture, the fruit split on the bottom. It might have started with a crack in the outer peel layer, and expanded quickly. It looks like a deep split, possibly going up into the membrane and that invites bacteria and insect invasion thus rotting the fruit rather quickly. I think I will let the experts...pro's help explain it better and add a more detailed explanation... I could learn more about it also..

I don't know either, but hopefully someone can help you!

I'm also located in the central valley up in NorCal where it can get pretty hot and dry.

With my inground "Scarlett Burgess" Mandarin i often get fruit splitting probably because it has a thin skin. This usually happens in the autumn when the rain starts suddenly after a dry summer. The way to avoid this is to ensure consistent watering, especially in the latter end of summer. This gradually stretches the skin of the juvenile fruit as it is growing to avoid too rapid expansion of the inside flesh as it swells with new rainwater, which in turn rips the skin apart.

This looks to be related to water. The fruits expands and contracts between day and night due to the change in temperature, especially more when closer to being ripe. If the tree is watered too frequently or it rains, this fluctuation can cause the fruits skin to break. Notice the break is on the underside of the fruit. So this is not caused by sun burn. Sometimes sprinklers that run too frequently and spraying water on the trees may be the cause