Artboards are greedy and try to suck in any layers that overlap them but are outside of their folder. You can prevent this with a button in the layer panel "lock" area... There's a button that looks like a little artboard called "Prevent auto-nesting in artboards".
If you check that box when you have an artboard layer selected, it prevents that artboard from auto-nesting. If you check it for a layer or layer-group, it prevents all artboards from grabbing that particular layer.