When you pulled the head off, normally there will be carbon buildup on the top of the piston, or some other sign of use. Was there one or more cyl's that seemed to be cleaner than the rest? this would signify that water (small amounts) was being introduced into the cyl. with the air/fuel mixture. and when the seal is lost, it doesn't take a very large hole for the burned gasses to go thru. Also, this could signify a cracked head. Have it checked.
Another thing, was there oil droplets in the radiator? If there was, then you had a cross between the pressurized oil passage and the water jacket. running, the oil will push into the water, when you shut it off, the pressurized water will push back into the oil galley.
