Whenever a mobile crane or a hook and ladder fire truck (the kind with the BIG ladder) needs to extend its boom or ladder out a long distance or with a heavy weight, it requires additional support to prevent it from toppling over. This support is provided by retractable outriggers.
Outriggers are sturdy metal legs that extend the footprint of the mobile equipment out to cover a larger area. They can either extend directly from the truck to the ground at an angle, or extend horizontally outward from the truck with a vertical leg that makes contact with the ground. In either case, the outriggers must lift all the weight of the truck up off the tires to provide maximum stability. Without the outriggers, the equipment is considered to be "on rubber" and with them it is "on outriggers". The operator must obey different loading restrictions in each case.
Stability is dependent on the equipment's footprint and center of mass. The footprint is the total area enclosed by the support structures of the equipment. The center of mass is the point at which the equipment would balance if it were set on top of a single point of support. If the center of mass is inside the footprint, the equipment is stable. If the center of mass is off the edge of the footprint, the equipment will topple.
So it's not hard to imagine how a crane could become unstable when extending its boom with a heavy weight hanging from it. Since not extending the boom isn't really an option, there are only two ways to provide extra stability: Use a counterweight to help shift the center of mass back over the footprint, or make the footprint bigger. All cranes have counterweights, but there is a limit to how heavy they can get. Too much and the crane wouldn't be able to move, and the ground would have trouble supporting it. Simply building the crane with a wider footprint would prevent it from traveling on public roads. Using retractable outriggers to extend the footprint therefore is essential to the crane's stability.
Some potential problems with outriggers:
No insulation from the ground - Outriggers are made of steel, and lift the rubber tires off the road. This provides a good electrical connection to ground. Particularly dangerous when working underneath overhead power lines.
Small contact patch - The outriggers are generally less than a foot squared each, meaning that there is a great deal of weight sitting on a relatively small surface. Soft surfaces such as dirt, mud, and even asphalt (for very heavy weights) will be damaged by the pressure. Not only would this damage need to be fixed, but outriggers sitting on damaged surfaces can't provide the support that they need to. Plywood sheets, two-by-fours, or other lumber is generally used in these cases to spread out the weight.
Forgetting to use them - It happens. Especially when the crane is being used in a narrow passage, such as a bridge, and the outriggers are in the way of other traffic. Some accidents have been attributed to the operator pulling in the outriggers to allow a truck to go through, and then forgetting to put them back out before resuming operation.
Outriggers not fully extended - Sometimes there is not enough room to extend the outriggers all the way. Sometimes a malfunction causes one or more outriggers to be deployed only part way or not at all. Sometimes the operator simply didn't extend them all the way for whatever reason. In any case, the footprint is not as big as it should be. The rated load for a crane on outriggers is calculated for when they are fully extended, and it can be difficult and dangerous to estimate what is safe for anything less than full extension.
Overloading - Overloading can mean two things. First, if the boom is extended too far with too much weight, the crane can still topple despite the extended footprint. Second, if the crane is loaded with too much weight but the center of mass is still inside the footprint, the outriggers themselves may collapse. If this happens, between the load now being shifted at an angle due to the collapse and the shortened footprint due to the broken outrigger, the center of mass may now be outside the footprint and topple the crane.
Cranes are carefully designed and their safe limits of operation are calculated by engineers who (hopefully) understand the responsibility they are entrusted with. Furthermore, crane operators are trained and skilled professionals, and licensing in the United States is available through various state agencies and The National Commission for the Certification of Crane Operators (http://www.nccco.org). Between good design practices and knowledgeable operators, crane accidents are thankfully rare events.