Overload (Also known as Overload Deformation, Brinelling, and True Brinelling) is where a bearing's raceways get permanent indentations at the ball pitch as a result of exceeding the static load limit of the raceway material. The indentations match the ball shape and are usually surrounded by a build-up formed by displaced material.
Freestyle skateboarding, by its very nature, does not limit itself to just rolling, so when we do Rail Tricks, we are applying axial forces as well. The static overloading and shock loads from rail tricks tend to cause overload deformation because it can easily exceed the axial load limit of bearings that were really only designed to withstand radial forces. Bearings with deep races, however, can be much more durable in this respect.
The ISO classification for this failure mode can be found in ISO 15243:2004 (Rolling bearings -- Damage and failures -- Terms, characteristics and causes), 5.5 Plastic deformation -> 5.5.2 Overload deformation.