Essentially, your DPF has a small fire in it and may melt down or otherwise become unrestorable. This occurs when too much soot accumulates for too long in the filter.
How do we avoid an uncontrolled regeneration?
Passive regeneration occurs during normal driving conditions when the exhaust temperature is high enough to oxidize the particulate matter trapped in the filter, for example on long haul highway runs.
In active regeneration, your engine deliberately increases exhaust temperatures to burn off the trapped particulates. This often involves injecting fuel into the exhaust system to raise temperatures. Active regeneration might be triggered automatically by your vehicle when needed, or you may start it yourself.
Regular DPF maintenance recommended by your manufacturer will prevent soot from accumulating to the point where an uncontrolled regeneration is possible. Check out the guarantees that you can get on your DPF service, and remember, at the DPF company, you only pay when it passes!