append is nesting divs when it shouldn't

append is nesting divs when it shouldn't

When I do something like 

$(".something").append($('<div class="aaa" /><div class="bbb" />')) 

the bbb div gets rendered inside the aaa div instead of them being siblings. Am I doing something wrong? It used to work but we've just upgraded to use the latest version of JQuery and it's stopped working.