I've done that before too, with vodka, starsan, and tapwater. It's caused by the beer cooling down and causing a suction.
The first time I used my fermentation refrigerator I hooked up a blowoff tube and put that in a gallon jug quarter full of starsan. I put the carboy in at 68 or so and set the temp controller for 60. By the time everything settled at 60 the next morning the blowoff tube had sucked the entire tube lenght or more of starsan into the carboy... Opps! Beer turned out fine.
Kegged: BCS American Pale Ale, Blonde Ale
Primary: BCS American Pale Ale
Bottled: Janet's Brown Ale, Dry Stout, Irish Red Ale, Apfelwein
Gone:American Pale Ale, Irish Red Ale, Dry Stout, West Coast Red
Long Gone: Too much