Why does fluid come out the trany dipstick

I got this 2 years ago as a Tow Away when I had my tow service, it's been stored at my brother's shop since. We pulled it out a month ago & have been doing some little repairs, brakes, oil change, etc. No trany service yet. When my brother went to move it last week ( he let run for about 10 minutes ) it spewed fluid out the trany dipstick about 1/2 cup he said, we'd like to know why. He's only seen this one other time in his 40 years working on vehicles. We've driven the car about 50 miles to help set the brake pads & shoes, then parked it. Why did it do this ? His mechanic friends not worry it's a Ford thing. Any help would be appreciated.