I think the loops pretty much always form. You just always get a sufficiently persistent hybrid contrail so you can see them. It likely requires quite precise atmospheric conditions - so as to maximize the hybrid contrail formation. More often the hybrid contrail does not extent into the connected part, so you get the "chromosome" appearance.
Here's a technical discussion of how the loops form. They are actually "vortex rings", a bit like smoke rings, formed by a kind of folding of the original vortices.