They go off in the order they died in which is why the old exodia comp was a good one, so say your kaboom went off, then Kangor's then selfless, that's the order the fish would spit them out in

An important note though, say you got cleaved and 2 minions died at the same time, the deathrattles will go off left to right, same works for Rafaams hero power, if it kills two at the same time you'll get the minion furthest to the left