I have yet to see this new "respawn on the commander" mechanic in play but it would be sensible if the commander had something like that horn and the ability to stand still and blow it uninterrupted for 5 seconds was what triggered the next spawn wave.
So rather than chasing the commander or fighting him and WHAM, 20 dudes spawn on your head, he has to stop and blow the horn. It would be like opening a gate -- you can do it while being beat on but it's not good for your health.
It would also provide fair warning to enemies that a new spawn wave is coming in.