Acid Mortar doesn't do Knockback, but the proc that is slotted is the Overwhelming Force Chance for Knockdown/Knockback to Knockdown. This gives Acid Mortar a chance to knock down enemies hit with it. Not sure how well that works, but that's the reasoning behind it.
Here's my Beam/Traps/Mace build, capped to everything but Psi and Melee, so very sturdy. I have recently begun liking Caltrops, which I originally skipped on this guy, so I may end up respeccing into it. Probably dropping Combat Jumping, or maybe Triage Beacon, though I like the regen of that power.