Baxi 552 soot problem?

The flames from the burner were lapping from under the enclosed casing, when I took the fire apart I found a massive build up of soot under the heat exchangers and all over the burner, I only cleand the boiler 3 months ago and everthing has been ok until today. there has been no soot fallen down the chimmly flu because the top of the exchanger are clean. I've cleaned out all the soot and everything seems to be running normal now. can anyone tell me why the soot build up has happened so fast and do I need to get a proffessional out. The boiler works fine now but I am concerned why is there so much soot almost a full dyson???