If you turn over a cart full of apples, you end up with apples everywhere, right? "Upset the apple cart" just means to upset things and cause disorder. "Things were running smoothly in the office until Paul upset the apple cart by changing the software." "Monica really upset the apple cart when she invited those skinheads to the NAACP meeting."


to create difficulty, to cause trouble

I don't want to upset the applecart now by asking you to change the date for the meeting.
another market question. most vegetables and produce were taken to market by a costermongers barrow .or cart.. sometimes the job of pulling the cart was given to the son .when the cart was loaded it was very heavy. and on occasions it would tip up and all the apples would spill out. so on loading the cry would be try not to upset the apple cart. meaning don't do more than you can manage
An apple cart was a street-side display of apples for sale. The most attractive displays would be an orderly, though somewhat precarious, stack.

Tipping the apple cart is turning order into chaos.