Changes in temperature will make the fuel expand and contract. As the fuel in the lines expands it'll push past the needle valves and into the air box. Get into the habit of turning off the petcock when leaving the bike for more than a a day or two. It also wouldn't hurt to check the needle valves for wear or dirt.
FYI you don't need to siphon out the air box, just take the rubber plugs off the drain tubes below the bike.
