Because the CSS transition: property is applied too broadly, a (generally invisible) animation happens after the classes have been set to quiescent state again by the carousel JavaScript code.

This doesn't matter, as the second animation is mostly invisible, except in some cases — For instance, since "transform" breaks z-index, the spurious animation needlessly doubles the time where z-index doesn't work in a parallax carousel (see https://codepen.io/domq/pen/ZoJrQZ )

Narrowing down the application of the transition: property to just the active sliding classes, helps (also demonstrated in the codepen, see comments under "CSS").