That actually happened to me this last fall when I dumped the air out of rear tires for snow chain install. I jacked the rear of the tractor up, rotated the tire so the valve steam is at the very top, pushed the core in a bit to dump what should be a very small mount of beat juice out of the valve stem. Then I removed the core....
Beat juice streamed out onto the ground until the air pressure dropped. I put the core back in to stop the flow, spraying juice all over. I rotated the tire around a bit, but when I removed the core again, it kept pouring out the juice! Very strange. I probably lost a few gallons onto the concrete.
Not sure how a liquid could flow up hill like that. Siphons do it, but I'd have a hard time believing that a siphon was created somehow. The other side worked as normal. Small amount of juice from the stem, removed the core and it was just air. Very strange indeed.