This issue ranges all over the interwebs and seems to affect many brands of cars. Solutions/causes range from spider/wasp webs in the vent line to gas-soaked charcoal canisters. Do a search and you'll see many solutions. Take your pick. If you are handy you can take apart your entire gas filling system and see what you find. One thing I've learned from reading about it is to NEVER top off while filling. When it clicks off, stop.