Parked Vehicle Lost Transmission Fluid

I am rebuilding my old 1966 Chevy Pickup. I installed a new 350 cid engine with a new automatic transmission.

I was having issues with my old gas tank so I wasn't driving it until I could afford a new tank. One day I noticed a lot of pink fluid on the ground underneath the truck.

Why would the transmission start to leak all of the sudden when I wasn't even using it?