Fridge Logic - Anything to do with Mr. Freeze falls in this category.
Ahaha. Fridge Logic. I get it.
Icy what you did there.
That pun leaves me cold.
Let's chill with the puns.
Poison Ivy was sitting in her chair/plant when she kisses Robin. She fights him and Batman. Then Batgirl comes in and they fight. Batgirl pushes her into the chair/plant which eats her... No reason was given as to why her chair just ate her...
One of two things happened: When simply lounging inside the plant, Poison Ivy didn't put enough pressure on its trigger mechanism to close it, but when she fell into it, the mechanism triggered and, like a perfect reflex action, the plant closed instinctively. Another possible reason is that Poison Ivy simply lost control of the plant due to being beaten up from the fight and her mind being otherwise occupied.
Ivy did also say "curses", which, judging by what she said next, which sounded a bit like "no, not 'close'", her plant obeyed what it thought was a command. That's how I understood it when I was younger, so I thought I'd just throw that out there.
Fridge Brilliance - Mr. Freeze is not having a moment of Bond Villain Stupidity at the beginning of the movie. His plan was to lure Batman (or even Robin) into a Friend or Idol Decision by freezing Robin and using that to cover his own retreat because his suit was running out of energy and his life would have been in danger if he had stuck around to fight Batman. In short, Mr. Freeze pulled a Batman Gambiton Batman!
The guy trying to save his wife saying "your emotions make you weak" definitely supports this theory