What does apple cart mean?

[Maddy]

English Language Tests, Intermediate level

ESL/EFL Test #424 "English Slang Idioms (163)", question 1

"Did you hear that the warehouse workers are trying to form a union at this company? That is sure to upset the ......... around here," Lisa told Linus.

(a) beanpole
(b) grocery
(c) water flow
(d) apple cart

English Language Tests, Intermediate level

ESL/EFL Test #424 "English Slang Idioms (163)", answer 1

"Did you hear that the warehouse workers are trying to form a union at this company? That is sure to upset the apple cart around here," Lisa told Linus.

Correct answer: (d) apple cart

Your answer was: incorrect
"Did you hear that the warehouse workers are trying to form a union at this company? That is sure to upset the water flow around here," Lisa told Linus.
_________________________

Hello,

Could you explain to me the meaning of apple cart?

[Our Tort System]

An apple cart is a cart loaded with apples.
The idiom though is "to upset the apple cart". Its meaning is explained here
_________________
Criminals love gun control - it makes their job safer.
--A patron saint of the Second Amendment.

[Maddy]

Thank you.