When Arnie's character is being tempted by the Devil to have his wife and daughter back, it switches to the scene, at Christmastime, when they were murdered. Arnie tries to save them, but can't, because they aren't physically real. But he leaps across the room in a panic, knocking over a Christmas tree... which isn't really there!
I think that the Devil can make some objects "real". If you remember Arnie takes an angel off the tree and then drops it on the floor where it smashes. So if the tree was real then, it stands to reason that he would tackle it by mistake and it would fall over ....don't you think?
Maybe he just wanted to make him look like a prat.......... or he just forgot to make the tree an illusion again..... if god is supposed to work in mysterious ways then I guess the devil can too.