How do you stop the Zombot Aerostatic Gondola from activating the fire traps?

The most annoying thing about fighting Dr. Zomboss at the end of the Lost City level is that he occasionally drops sandbags on the fire traps, wiping out a whole lane of plants (except for extremely tough ones, like an Endurian that's been fed a plant food). In many playthroughs of the stage, I've occasionally seen him miss the trap, dropping the sandbag on the adjacent square instead, but I've never been able to determine the circumstances that led to it.

At the front of the gondola is a zombie figurehead with some vines hanging out of its mouth. My best guess is that something can pull the ship forward by the vines and make Zomboss miss the trap, but I can't figure out what it is. I've given plant food to each type of available plant in the vicinity of the ship as he starts his fire trap move, but none of them pull the ship forward.

The only plausible theory I have is that this just happens by random chance sometimes, to give the player a bit of a break. But Zomboss's particularly devastating moves can be interrupted in most if not all other battles, which makes me think there must be a way to do it in this one too. Is there?