To illustrate the problem, let me give you an example.

Suppose we have an HTML element:

<div class="lozad animation" data-background-image="URL">
    some html
</div>

Now lets hang two lozad call on the one element:

// first default call
lozad().observe()

// second - custom call
const el = document.querySelector('.animation')
lozad( el, {
    load: function(){
        // some code for animation
    }
} ).observe() 

For both of these calls the same check is triggered to see if the lozad already loaded. So only the first call will work, and on the second call lozad will see that it is already done and just skip execution of our custom function.

Also!

It is betted to specify the unique name for the data-loaded html attribute, bacause it is a rather common name that can be used by another script and if that happens we catch unpredictable conflict. Therefore, it is better to give a more unique name, such as data-lozaded.