This will be the ECU and engine attempting to regenerate the DPF by adding more fuel to the engine, but it's a system that sometimes does not work properly. It is absolutely vital that you regularly check the oil level in the engine. If it rises above the full mark, then this is very dangerous because a compression ignition diesel engine can start to run uncontrollably on its sump oil and could blow up. Wise to get it into a dealer for a proper check and an oil change because the contaminated oil won't be doing the engine any good anyway.