I had a problem with mine doing this the other day from mx racing and roough trail ridding. Mine came from splash out of the overflow and not overheating. I reduced the fill level of the overflow from the full mark to about 1/4 inch above the low mark and it stopped. I made sure the radiator was full also.
Mine does it as well and I ride trails really hard. I believe it is coming from the overflow valve as it just started doing it on mine when I had the coolant level filled to the full mark. I would check for any obvious leaks, but if the bike is not overheating, I wouldn't worry about it.