Spontaneous coolant discharge

[xdab]

Quick ride to warm up 500 RR-S for oil change. While idling in neutral upon return bike started vigorously leaking coolant. Shut down bike and inspected expecting to find loosened/cracked hose. Nothing loose, cracked or otherwise. Started bike up again to see where coming from. Nowhere. Didn't do it again. Have ridden several time since with no repeat episode. Since there are no connection or broken issues I'm assuming that discharge came from overflow hose. Was a pretty aggressive and sustained flow though...not a dribble. Was coming out forcefully and probably would have drained out rads if I didn't shut it off.

Question....what would cause there to be such a strong pissing out of overflow that just decided to happen randomly and why can't I get it to repeat? Bike has been a lot hotter than that before without any drama so not simply case of bike boiling over I wouldn't think. Some kind of vacuum phenomenon? I can certainly keep an eye on it and see if it does it again but don't want to necessarily see it happen again 20 miles from camp. Want to get on top of it.

Any theories?

[aj_mariner]

Filled radiators to the tip top?

[twowheels]

Thermostat issues most likely.