The most likley cause is your the oil is over full. (dont ask me how I know this

) This hose is an engine breather tube. If the oil is not over full, have you stored the bike on its tale? have you rolled the bike latley? if none of the above you possble have a great deal of blow-by (Piston rings are shot) If the oil is over full, check the air filter for oil and drain and clean the oil out of the air box.
To check the oil: Start the bike, run for a few moments, unscrew the oil cap, wipe off the dip stick, screw cap in and back out again and chek the oil level. If overfull drain some oil from the oil Reservoir using a 9/16 or 14mm socket.
hope this helps,
SJ