To the Brits, pickle is a relish sauce. And that is closer to the original meaning of the word. Pickle is a brine sauce that was once slathered over meats back in the old, old days of yore.
This can more easily explain the idiom ‘In a pickle’. If you’ve ever been trapped between two bases in a baseball game you know how uncomfortable pickles are. The metaphor is quite clear if you think about it in a relish sort of way. You are in a quagmire of salty liquid about to be eaten.