Merriam-Webster, and his hero, Peter Sokolowski, host of the Merriam-Webster Word of the Day podcast, would say "blame it on the Latin."
In this case the Late Latin word casualis
"by chance," from Latin casus "chance, occasion, opportunity; accident, event"
It's the lack of planning that casual and casualty have in common.
Merriam loves the chance to show off how much he knows. 😊