A lot of oil was thrown to air filter box. Why?

On my last ride after 15 miles I saw there was a lot of oil in air filter box. Oil was also all around the back of a bike on right panniers... total mess. I don't think that there was too much oil in machine and only the excessice oil was thrown out because after that I did an oil level chech and there was only on the bottom of a measure stick shown some oil left. I have bullet 500, 5 speed iron barrel, 2006. Do you have any ideas what is wrong?