IMO, the spiders are just the straw that breaks these little kids' backs.
There is a big bug up on the screen and he's mad at us. He menacingly holds out a can of stuff and then depresses the top. Stuff starts to come out - it is "smoke." (Although it isn't really smoke from something burning, but some sort of moisture thing, the effect is the same.) The theater fills with this stuff being shot out by the big, angry bug.
In some parts of the theater, it is just all around you. In other parts, you can't see your hand in front of your face...much less Mommy or Daddy. And you're all alone, lost in that stuff the big, angry bug sent. You can feel Mommy and Daddy if they touch you, but you can not see them through the fog.
As if this weren't bad enough, loud and ominous music begins thumping. Colored lights flash on and off. It is all very scary and creepy, if you're three years old.
Then giant spiders fall down from who knows where. Their bodies are about the size of your face, if you're three. They are bouncing up and down. Who knows when they'll stop bouncing and attack you.
If you're three, it's time to scream.
This is likely why almost every time I'm there, some parent dashes out with a toddler screaming bloody murder.