A lot of things can cause a broken axle, but some of the most common causes depend on what type of drivetrain your car has:
Front-wheel-drive vehicles have a rubber covering over the CV joint known as a CV boot. If the boot tears, debris from the road gets into the joint and damages both the CV joint and the half shaft.
On rear-wheel-drive or all-wheel-drive vehicles, the same principle applies, but it’s usually the axle seals that fail and begin to leak fluid.
In addition to these mechanical failures, axles can also break from hitting potholes or larger accidents.