Kawasaki vulcan 1500 why does coolant leak out of overflow hose?

# Airlock or blockage in the system # Overfilled system # Damaged/broken pressure cap or seal # Thermostat faulty # Head gasket blown Check 1 to 4 first but if there is water in the oil, or oil in the water (whitish froth inside the filler cap), or a misfire, or difficult starting or white smoke out of the back, normally its 5.