Secondary attack is right click - Archon works fine on right click. I am unsure why it doesn't behave as expected when using left click.
I've just tested this in game with my wizard, if you bound the Archon ability to your primary attack:
your Primary attack button (left click) and then hold shift and click on the ground it will activate Archon form.
clicking on an enemy with Archon bound to your Primary attack button will not trigger Archon form.
However, for additional testing, I put Mirror Images/Slow Time on my Primary attack button. Upon clicking the ground I moved as I expected, but upon clicking an enemy the ability was used.
From my testing it just appears to be some weird behaviour with Archon, there may be some built in fail safe to stop you from triggering long cool downs (Archon is our only long cool down) when trying to move maybe?