The answer is quite simply general abuse. The limited-slip differential in your rear-wheel-drive car was put there to provide additional grip and some slidey action, but that doesn't mean you should powerslide around every corner. Not only is it dangerous, but it tends to lead to overheating. On the flip side, you need to give the differential some time to warm up before giving it the beans. The first-generation Mercedes-AMG CLK Black famously came with a warning letter telling the driver to do this. When it comes to 4x4s, differentials are usually damaged by sharp rocks. Investing in proper underbody protection before off-roading is an absolute must-have off-road modification.