Hmmm. That long in 2nd, with a pretty good load (loose sand has a LOT of rolling resistance), may have heated the tranny juice too hot and foamed it. Contributing factors are high ambient heat and no converter lockup (converter efficiency losses are the main heat producer) in 2nd gear. Like Kevin, I say look at the fluid for burning (brown stain) and a burnt smell. If you want to know what the burnt smell is like, drip a drop or two of atf on a hot exhaust manifold.

Not responsible for advice not taken...