I can't tell how exactly they got there, as I wasn't present to witness it. They made my traps to glitch as well, as they stay locked onto them and don't fire at other enemies. I can only hope to quickly dispatch them with Lighting.
Correct Lee, it was the Blade Lotus Knock Back, which was able to shift units first above and then in chasm.
Will be fixed next Patch :)
I managed to suceessfully replicate this. It's quite interesting actually.
So from what I can tell this is a result of some behaviour regarding the blade lotus. I believe that if a blade lotus is triggered in a way that would push a unit into a solid obstruction it is possible that those units will sink into the ground.
For example I had a similar setup but had three blade loti across the entrance area, these were triggered when enemy units walked on them. Blade lotus would push the unit into another active blade lotus and the units would instantly sink into the ground.
Still occurring in Patch 1.4.3 RC. Re-opening.